Support deleting a namespace by admins

[netomi]

Right now you can only delete namespace manually via sql statements which is tedious and error-prone.

[netomi]

actually I found out that there is a job to delete orphaned namespaces, i.e. namespaces without a member.

[autumnfound]

Question: Do we want to use a scheduled deletion like some of the other operations, like the namespace move, or just do it at request time?

Additionally, for bundles and dependencies, I'm assuming we want to remove those as part of the clean up as well?

[netomi]

changing the namespace is heavy as it involves file operations.

For the start, I would include a check if the namespace has any extensions and if that is the case, fail.
We should only allow empty namespaces to be deleted.

[netomi]

Fixed by #1773.