Admin interface: extract admins from admin group
Adjust new admin handling: simpler is_admin?, add fkey
Admin interface: rename SplashscreenSet to TeaserSet

Conflicts:
	db/personas.data.psql
ui: js: clipboard: don't parseInt() UUIDs

this lead the clipboard to load images such as `/media_resources/NaN/images` (if UUID starts with character, doesn't work at all);
and `/media_resources/2/images` – using just the first numbers of a UUID, which led to "not authorised" flashes (even worse)
ui: set: resources view: js: when saving the display setting, reset query params
Admin interface: improve workflow of managing meta key definitions