Feature: Admin can set a user's trust level

An admin can set the user's trust level - note that they can only upgrade their trust level.

In order to maintain the "normal promotion train" following an admin upgrade, a flag is set on the user indicating that their trust level has been set by an admin, and they are promoted to the next level when they satisfy the difference of the requirements between the two trust levels.

Attention: @samsaffron 

(Can squash commits as necessary)