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/dump.sql

@@ -854,6 +854,7 @@ CREATE TABLE users (
     github_com_user_id bigint,
     hashed_one_time_passcode bytea,
     one_time_passcode_expires_at timestamp with time zone,
+    is_system boolean DEFAULT false NOT NULL,
     CONSTRAINT one_time_passcode_set CHECK ((((hashed_one_time_passcode IS NULL) AND (one_time_passcode_expires_at IS NULL)) OR ((hashed_one_time_passcode IS NOT NULL) AND (one_time_passcode_expires_at IS NOT NULL))))
 );
 
@@ -867,6 +868,8 @@ COMMENT ON COLUMN users.hashed_one_time_passcode IS 'A hash of the one-time-pass
 
 COMMENT ON COLUMN users.one_time_passcode_expires_at IS 'The time when the one-time-passcode expires.';
 
+COMMENT ON COLUMN users.is_system IS 'Determines if a user is a system user, and therefore cannot login or perform normal actions';
+
 CREATE VIEW group_members_expanded AS
  WITH all_members AS (
          SELECT group_members.user_id,
@@ -892,6 +895,7 @@ CREATE VIEW group_members_expanded AS
     users.quiet_hours_schedule AS user_quiet_hours_schedule,
     users.name AS user_name,
     users.github_com_user_id AS user_github_com_user_id,
+    users.is_system AS user_is_system,
     groups.organization_id,
     groups.name AS group_name,
     all_members.group_id