Reinstate `push` and `admin` scopes

Reinstate push and admin scopes (temporarily reverted until the BE fix which is now merged: pleroma!2061 (merged)).

See !1032 (comment 48084)

Note: right now BE does not enforce admin scopes usage, and FE will work with older and new versions of BE without the need to do anything to tokens (e.g. sign out / sign in isn't needed).

NB. As for the push scope, it controls accessibility of push subscriptions API endpoints, see https://git.pleroma.social/pleroma/pleroma/blob/develop/lib/pleroma/web/mastodon_api/controllers/subscription_controller.ex.