This changes the content of `@categories_lookup` from `Category` objects
to IDs since the category names aren't needed anymore. The lookup
method has been renamed too.
Changed internals so trust levels are referred to with
TrustLevel[1], TrustLevel[2] etc.
This gives us much better flexibility naming trust levels, these names
are meant to be controlled by various communities.
PERF: disabled refresh_avatar callback when importing users
PERF: avoid using UsernameSuggester when not needed
FIX: categories wasn't working
FIX: posts from deleted users are now from the system user