We hit some nasty issues where uninitializing an extension failed hard,

preventing all future attempts at uninitializing. These had to do with

the dynamically-set/removed extension.admin_site attribute, which was

assumed to be available if has_admin_site was True.
We now check for the existence of admin_site instead, and also catch

attempts at removing non-existing URLs in DynamicURLResolver.

Unit tests passed.