Add deletion to import

[Boldewyn]

On import delete bookmarks, that were deleted in the external service. Make this configurable.

For this to work we must track, which URL was imported from where, and check if it's gone on new import. It gets nasty, when a user imports from more than one source.

This is low prio and assumably a WONTFIX, especially since we have the functionality already the other way round.