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/queries/users.sql

@@ -11,7 +11,9 @@ SET
 		'':: bytea
 	END
 WHERE
-	id = @user_id RETURNING *;
+	id = @user_id
+	AND NOT is_system
+RETURNING *;
 
 -- name: GetUserByID :one
 SELECT
@@ -46,7 +48,8 @@ SELECT
 FROM
 	users
 WHERE
-	deleted = false;
+	deleted = false
+  	AND CASE WHEN @include_system::bool THEN TRUE ELSE is_system = false END;
 
 -- name: GetActiveUserCount :one
 SELECT
@@ -54,7 +57,8 @@ SELECT
 FROM
 	users
 WHERE
-	status = 'active'::user_status AND deleted = false;
+	status = 'active'::user_status AND deleted = false
+	AND CASE WHEN @include_system::bool THEN TRUE ELSE is_system = false END;
 
 -- name: InsertUser :one
 INSERT INTO
@@ -223,6 +227,11 @@ WHERE
 			created_at >= @created_after
 		ELSE true
 	END
+  	AND CASE
+  	    WHEN @include_system::bool THEN TRUE
+  	    ELSE
+			is_system = false
+	END
 	AND CASE
 		WHEN @github_com_user_id :: bigint != 0 THEN
 			github_com_user_id = @github_com_user_id
@@ -316,15 +325,17 @@ UPDATE
     users
 SET
     status = 'dormant'::user_status,
-	updated_at = @updated_at
+    updated_at = @updated_at
 WHERE
     last_seen_at < @last_seen_after :: timestamp
     AND status = 'active'::user_status
+		AND NOT is_system
 RETURNING id, email, username, last_seen_at;
 
 -- AllUserIDs returns all UserIDs regardless of user status or deletion.
 -- name: AllUserIDs :many
-SELECT DISTINCT id FROM USERS;
+SELECT DISTINCT id FROM USERS
+	WHERE CASE WHEN @include_system::bool THEN TRUE ELSE is_system = false END;
 
 -- name: UpdateUserHashedOneTimePasscode :exec
 UPDATE