This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
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
