Add "delete/relabel" for editors

[mzur]

Users asked to be able to only attach labels to existing annotations, even if there are existing labels of the own user which would normally be replaced by Largo. We could add this feature as a new "delete/relabel" switch (similar to the one for project experts). The switches could be called "delete/relabel own" and "delete/relabel other" or something like that. The "own" switch is enabeld by default so the default Largo behavior is the same than before.