Added 'admin' & 'push' OAuth scopes

AdminFE support for pleroma!2025 (merged)

PleromaFE support for admin scope (required for "Login with PleromaFE" feature): pleroma-fe!1024 (merged)

Note: push scope is also added since it's a standard scope, and AdminFE should be full-featured. Open to removing it if there are arguments against it.

Note: disabled PleromaFE login feature since PleromaFE shouldn't request admin scope.