Delete tags when there is no tagging associated

[chargio]

If there are no taggins associated, the tags should be deleted.

Options:
When the tagging is deleted
Creating a task that do so and can be executed every night

Requirements:
As some tags can be created by admins and are needed independently of the number of usages, it should be possible to mark some tags as admin_created (or dont_delete), so they are kept when the task is run or the tagging is deleted.