When creating a content type, also install it in the module.

This helps django-admin-tools which directly looks up model paths and
crashes if it can't find some model (arguably a bug).
Use specific logger so one can pinpoint the origin of log entries.
Better permission checks in the tree editor and item editor.

Filter out available content types according to the user's add permission.
Cache permission names for efficiency.
Only add delete button in item_editor JS if we actually can delete ct item.
Only add add button and drag handle in tree_editor if user can add/change items.
Verify that user may delete entries in tree_editor "delete" action.
Item_editor "add ct" dropdown only offers cts that user can actually add.

There probably are lots of holes in the permission checks still, so caveat emptor.