Permissions plugins listens now for album_imported and item_imported
Tests: fix unload_plugins

Don't mutate the base classes. This was causing plugins to break on the second
test that uses them, since after they were unloaded their class-level fields
were broken (since the modules are not re-loaded).

This makes it more clear than ever that we need to encapsulate plugin loading
state using some manner of PluginManager.