feat: implement sign up with GitHub for the first user (#16629)

Second PR to address https://github.com/coder/coder/issues/16230. See
the issue for more context and discussion.

It adds a "Continue with GitHub" button to the `/setup` page, so the
deployment's admin can sign up with it. It also removes the "Username"
and "Full Name" fields to make signing up with email faster. In the
email flow, the username is now auto-generated based on the email, and
full name is left empty.

<img width="1512" alt="Screenshot 2025-02-21 at 17 51 22"
src="https://github.com/user-attachments/assets/e7c6986b-c05e-458b-bb01-c3aea3b74c0e"
/>

There's a separate, follow up issue to visually align the `/setup` page
with the new design system: https://github.com/coder/coder/issues/16653

coderd/userauth_test.go

@@ -22,6 +22,7 @@ import (
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
+	"go.uber.org/atomic"
 	"golang.org/x/oauth2"
 	"golang.org/x/xerrors"
 
@@ -254,11 +255,20 @@ func TestUserOAuth2Github(t *testing.T) {
 	})
 	t.Run("BlockSignups", func(t *testing.T) {
 		t.Parallel()
+
+		db, ps := dbtestutil.NewDB(t)
+
+		id := atomic.NewInt64(100)
+		login := atomic.NewString("testuser")
+		email := atomic.NewString("testuser@coder.com")
+
 		client := coderdtest.New(t, &coderdtest.Options{
+			Database: db,
+			Pubsub:   ps,
 			GithubOAuth2Config: &coderd.GithubOAuth2Config{
 				OAuth2Config:       &testutil.OAuth2Config{},
 				AllowOrganizations: []string{"coder"},
-				ListOrganizationMemberships: func(ctx context.Context, client *http.Client) ([]*github.Membership, error) {
+				ListOrganizationMemberships: func(_ context.Context, _ *http.Client) ([]*github.Membership, error) {
 					return []*github.Membership{{
 						State: &stateActive,
 						Organization: &github.Organization{
@@ -266,16 +276,19 @@ func TestUserOAuth2Github(t *testing.T) {
 						},
 					}}, nil
 				},
-				AuthenticatedUser: func(ctx context.Context, client *http.Client) (*github.User, error) {
+				AuthenticatedUser: func(_ context.Context, _ *http.Client) (*github.User, error) {
+					id := id.Load()
+					login := login.Load()
 					return &github.User{
-						ID:    github.Int64(100),
-						Login: github.String("testuser"),
+						ID:    &id,
+						Login: &login,
 						Name:  github.String("The Right Honorable Sir Test McUser"),
 					}, nil
 				},
-				ListEmails: func(ctx context.Context, client *http.Client) ([]*github.UserEmail, error) {
+				ListEmails: func(_ context.Context, _ *http.Client) ([]*github.UserEmail, error) {
+					email := email.Load()
 					return []*github.UserEmail{{
-						Email:    github.String("testuser@coder.com"),
+						Email:    &email,
 						Verified: github.Bool(true),
 						Primary:  github.Bool(true),
 					}}, nil
@@ -283,8 +296,23 @@ func TestUserOAuth2Github(t *testing.T) {
 			},
 		})
 
+		// The first user in a deployment with signups disabled will be allowed to sign up,
+		// but all the other users will not.
 		resp := oauth2Callback(t, client)
+		require.Equal(t, http.StatusTemporaryRedirect, resp.StatusCode)
 
+		ctx := testutil.Context(t, testutil.WaitLong)
+
+		// nolint:gocritic // Unit test
+		count, err := db.GetUserCount(dbauthz.AsSystemRestricted(ctx))
+		require.NoError(t, err)
+		require.Equal(t, int64(1), count)
+
+		id.Store(101)
+		email.Store("someotheruser@coder.com")
+		login.Store("someotheruser")
+
+		resp = oauth2Callback(t, client)
 		require.Equal(t, http.StatusForbidden, resp.StatusCode)
 	})
 	t.Run("MultiLoginNotAllowed", func(t *testing.T) {
@@ -988,6 +1016,7 @@ func TestUserOIDC(t *testing.T) {
 		IgnoreEmailVerified bool
 		IgnoreUserInfo      bool
 		UseAccessToken      bool
+		PrecreateFirstUser  bool
 	}{
 		{
 			Name: "NoSub",
@@ -1150,7 +1179,17 @@ func TestUserOIDC(t *testing.T) {
 				"email_verified": true,
 				"sub":            uuid.NewString(),
 			},
-			StatusCode: http.StatusForbidden,
+			StatusCode:         http.StatusForbidden,
+			PrecreateFirstUser: true,
+		},
+		{
+			Name: "FirstSignup",
+			IDTokenClaims: jwt.MapClaims{
+				"email":          "kyle@kwc.io",
+				"email_verified": true,
+				"sub":            uuid.NewString(),
+			},
+			StatusCode: http.StatusOK,
 		},
 		{
 			Name: "UsernameFromEmail",
@@ -1443,6 +1482,15 @@ func TestUserOIDC(t *testing.T) {
 			})
 			numLogs := len(auditor.AuditLogs())
 
+			ctx := testutil.Context(t, testutil.WaitShort)
+			if tc.PrecreateFirstUser {
+				owner.CreateFirstUser(ctx, codersdk.CreateFirstUserRequest{
+					Email:    "precreated@coder.com",
+					Username: "precreated",
+					Password: "SomeSecurePassword!",
+				})
+			}
+
 			client, resp := fake.AttemptLogin(t, owner, tc.IDTokenClaims)
 			numLogs++ // add an audit log for login
 			require.Equal(t, tc.StatusCode, resp.StatusCode)
@@ -1450,8 +1498,6 @@ func TestUserOIDC(t *testing.T) {
 				tc.AssertResponse(t, resp)
 			}
 
-			ctx := testutil.Context(t, testutil.WaitShort)
-
 			if tc.AssertUser != nil {
 				user, err := client.User(ctx, "me")
 				require.NoError(t, err)