Document editor priorities

If several document editors are enabled and the user has not set a preferred document editor, the document editor is selected somehow randomly. This happens also in read-only mode if ONLYOFFICE is enabled without editing. ONLYOFFICE should be the last fallback.

So we priorise the different edtors. The order is (hight to low priority)

1. OpenOlat HTML editor
2. Office for the web
3. Collabora Office
4. ONLYOFFICE