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/users.go

@@ -118,6 +118,8 @@ func (api *API) firstUser(rw http.ResponseWriter, r *http.Request) {
 // @Success 201 {object} codersdk.CreateFirstUserResponse
 // @Router /users/first [post]
 func (api *API) postFirstUser(rw http.ResponseWriter, r *http.Request) {
+	// The first user can also be created via oidc, so if making changes to the flow,
+	// ensure that the oidc flow is also updated.
 	ctx := r.Context()
 	var createUser codersdk.CreateFirstUserRequest
 	if !httpapi.Read(ctx, rw, r, &createUser) {
@@ -198,6 +200,7 @@ func (api *API) postFirstUser(rw http.ResponseWriter, r *http.Request) {
 			OrganizationIDs: []uuid.UUID{defaultOrg.ID},
 		},
 		LoginType:          database.LoginTypePassword,
+		RBACRoles:          []string{rbac.RoleOwner().String()},
 		accountCreatorName: "coder",
 	})
 	if err != nil {
@@ -225,23 +228,6 @@ func (api *API) postFirstUser(rw http.ResponseWriter, r *http.Request) {
 		Users: []telemetry.User{telemetryUser},
 	})
 
-	// TODO: @emyrk this currently happens outside the database tx used to create
-	// 	the user. Maybe I add this ability to grant roles in the createUser api
-	//	and add some rbac bypass when calling api functions this way??
-	// Add the admin role to this first user.
-	//nolint:gocritic // needed to create first user
-	_, err = api.Database.UpdateUserRoles(dbauthz.AsSystemRestricted(ctx), database.UpdateUserRolesParams{
-		GrantedRoles: []string{rbac.RoleOwner().String()},
-		ID:           user.ID,
-	})
-	if err != nil {
-		httpapi.Write(ctx, rw, http.StatusInternalServerError, codersdk.Response{
-			Message: "Internal error updating user's roles.",
-			Detail:  err.Error(),
-		})
-		return
-	}
-
 	httpapi.Write(ctx, rw, http.StatusCreated, codersdk.CreateFirstUserResponse{
 		UserID:         user.ID,
 		OrganizationID: defaultOrg.ID,
@@ -1351,6 +1337,7 @@ type CreateUserRequest struct {
 	LoginType          database.LoginType
 	SkipNotifications  bool
 	accountCreatorName string
+	RBACRoles          []string
 }
 
 func (api *API) CreateUser(ctx context.Context, store database.Store, req CreateUserRequest) (database.User, error) {
@@ -1360,6 +1347,13 @@ func (api *API) CreateUser(ctx context.Context, store database.Store, req Create
 		return database.User{}, xerrors.Errorf("invalid username %q: %w", req.Username, usernameValid)
 	}
 
+	// If the caller didn't specify rbac roles, default to
+	// a member of the site.
+	rbacRoles := []string{}
+	if req.RBACRoles != nil {
+		rbacRoles = req.RBACRoles
+	}
+
 	var user database.User
 	err := store.InTx(func(tx database.Store) error {
 		orgRoles := make([]string, 0)
@@ -1376,10 +1370,9 @@ func (api *API) CreateUser(ctx context.Context, store database.Store, req Create
 			CreatedAt:      dbtime.Now(),
 			UpdatedAt:      dbtime.Now(),
 			HashedPassword: []byte{},
-			// All new users are defaulted to members of the site.
-			RBACRoles: []string{},
-			LoginType: req.LoginType,
-			Status:    status,
+			RBACRoles:      rbacRoles,
+			LoginType:      req.LoginType,
+			Status:         status,
 		}
 		// If a user signs up with OAuth, they can have no password!
 		if req.Password != "" {
@@ -1437,6 +1430,10 @@ func (api *API) CreateUser(ctx context.Context, store database.Store, req Create
 	}
 
 	for _, u := range userAdmins {
+		if u.ID == user.ID {
+			// If the new user is an admin, don't notify them about themselves.
+			continue
+		}
 		if _, err := api.NotificationsEnqueuer.EnqueueWithData(
 			// nolint:gocritic // Need notifier actor to enqueue notifications
 			dbauthz.AsNotifier(ctx),