improving feedback on delete of vocab

[nvdk]

we should improve delete so that it only removes the vocabulary when it's completely removed

1. delete of a vocab should create a task for the deletion
2. the status of the vocab should be changed to deleting (so it can be locked in the interface)
3. a deletion service picks up on the task and removes all related entities from the db via mu-auth
4. when this is done a polling request is started to check counts in mu-search, when it reaches 0 the vocab is deleted and the task is successful
5. if however the timeout is reached, the deletion is set to failed and the admin is notified in the UI

[marc-portier]

adding to this maybe my recent experience with the vocab-delete:

as a consequence of these errors (the actual back-end cause is still under investigation) at least the user-interface should be able to give some solid feedback to the end user -- and not (like now) just keep that dialogue open eternally ....

[MikiDi]

related to #63