Closes #51153

#43276 introduced a better way to assign user roles on a Sourcegraph instance. Before #43276, we only had two types of uses:

• Site Admins
• Non-Site Admins

With a proper RBAC system in place, we can have different types of roles assigned to a user; however, we've been relying on the site_admin column on the users table, which isn't the right way to determine the SiteAdmin status of a user.

In this PR, I've removed all references to the users.site_admin column and ensured that site_admin status is computed based on the user's roles. This PR doesn't remove the site_admin column from the users table (this will be done in a follow-up PR).

The query for fetching user information has been updated to reference the user_roles and roles table to figure out the site_admin status of a user. That means the query for fetching a user changed from

SELECT  
	id, 
	username, 
	avatar_url, 
	deleted_at, 
	site_admin 
FROM users 
WHERE deleted_at IS NULL;

to

SELECT 
	id, 
	username, 
	avatar_url, 
	deleted_at, 
	EXISTS(
		SELECT 1 FROM user_roles 
		JOIN roles ON roles.id = user_roles.role_id 
		WHERE user_roles.user_id = users.id AND roles.name = 'SITE_ADMINISTRATOR'
	) AS site_admin 
FROM users 
WHERE deleted_at IS NULL;

This ensures we remove the dependency on users.site_admin.

Test plan

• Ensure all existing tests are passing.
• Temporarily removed the users.site_admin column and ensured tests were passing.
• Manually tested on my local instance.

Follow-up:

There'll be a follow-up PR to remove the users.site_admin column in the next non-patch release.