This change both speeds up specs (less strings to allocate) and helps catch
cases where methods in Discourse are mutating inputs.
Overall we will be migrating everything to use #frozen_string_literal: true
it will take a while, but this is the first and safest move in this direction
- Adds the advanced option to accept email from non-users per category email-address
- Adds tests covering the new feature
- Adds UI to configure this feature in the frontend
