chore: add prebuilds system user (#16916)

Pre-requisite for https://github.com/coder/coder/pull/16891

Closes https://github.com/coder/internal/issues/515

This PR introduces a new concept of a "system" user.

Our data model requires that all workspaces have an owner (a `users`
relation), and prebuilds is a feature that will spin up workspaces to be
claimed later by actual users - and thus needs to own the workspaces in
the interim.

Naturally, introducing a change like this touches a few aspects around
the codebase and we've taken the approach _default hidden_ here; in
other words, queries for users will by default _exclude_ all system
users, but there is a flag to ensure they can be displayed. This keeps
the changeset relatively small.

This user has minimal permissions (it's equivalent to a `member` since
it has no roles). It will be associated with the default org in the
initial migration, and thereafter we'll need to somehow ensure its
membership aligns with templates (which are org-scoped) for which it'll
need to provision prebuilds; that's a solution we'll have in a
subsequent PR.

---------

Signed-off-by: Danny Kopping <dannykopping@gmail.com>
Co-authored-by: Sas Swart <sas.swart.cdk@gmail.com>

coderd/database/modelmethods.go

@@ -423,6 +423,7 @@ func ConvertUserRows(rows []GetUsersRow) []User {
 			AvatarURL:      r.AvatarURL,
 			Deleted:        r.Deleted,
 			LastSeenAt:     r.LastSeenAt,
+			IsSystem:       r.IsSystem,
 		}
 	}