Calculate ultimateGear achievement on user.save

This does several things:
- Moves the calculation of the ultimateGear achievement into its on
function on user.ops.
- Calls user.ops.ultimateGear on user.save instead of on item purchase
- Fixes bug where ultimateGear was not awarded if user wasn't wearing
their class gear at the time of item purchase.
- Adds content.classes and content.gearTypes which we should start using
elsewhere.

Fixes habitrpg/habitrpg#2232.