Pre-requisite for#16891

Closescoder/internal#515

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

Our data model requires that all workspaces have an owner (ausers 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 approachdefault hidden here; in other words, queries for users will by defaultexclude 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 amember 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.