We want to put the name of the trust level in to generated URLs, not the human-readable form.
i.e.:
`/admin/users/list/newuser`
rather than:
`/admin/users/list/new user`
Refactors `TrustLevel` and moves translations from server to client
Additional changes:
* "staff" and "admin" wasn't translatable in site settings
* it replaces a concatenated string with a translation
* uses translation for trust levels in users_by_trust_level report
* adds a DB migration to rename keys of translation overrides affected by this commit
