feat: implement 'is_default' org field (#12142)

The first organization created is now marked as "default". This is
to allow "single org" behavior as we move to a multi org codebase.

It is intentional that the user cannot change the default org at this
stage. Only 1 default org can exist, and it is always the first org.

Closes: https://github.com/coder/coder/issues/11961

coderd/database/migrations/000193_default_organization.down.sql

@@ -0,0 +1,2 @@
+DROP INDEX organizations_single_default_org;
+ALTER TABLE organizations DROP COLUMN is_default;

coderd/database/migrations/000193_default_organization.up.sql

@@ -0,0 +1,16 @@
+-- This migration is intended to maintain the existing behavior of single org
+-- deployments, while allowing for multi-org deployments. By default, this organization
+-- will be used when no organization is specified.
+ALTER TABLE organizations ADD COLUMN is_default BOOLEAN NOT NULL DEFAULT FALSE;
+
+-- Only 1 org should ever be set to is_default.
+create unique index organizations_single_default_org on organizations (is_default)
+	where is_default = true;
+
+UPDATE
+	organizations
+SET
+	is_default = true
+WHERE
+	-- The first organization created will be the default.
+	id = (SELECT id FROM organizations ORDER BY organizations.created_at ASC LIMIT 1 );