Feature: Admin can modify user trust levels

Resubmitting following on from discussion in https://github.com/discourse/discourse/pull/1122 and [from this post on meta](http://meta.discourse.org/t/feature-mod-manually-sets-a-users-trust-level/8021) 

Attn: @samsaffron

Note: I have added to the `en` locale files as part of this PR but not to any of the other locale files - how does this happen?