Add the ability for an admin to ban a user from the admin interface

[katska]

Problem we're trying to solve
Although thankfully fairly rare so far, there are occasions when someone persistently abuses Planning Alerts comments. A recent example was highlighted when someone reported that someone impersonated other people in multiple comments to a local council (who reported this to Planning Alerts). There might also be other reasons for banning someone such as abusing the API or using the service against the terms and conditions.

Idea
There should be a button in the admin interface that allows the admin to both ban that email address from the site, send that user an appropriate message, and also the contact the planning authority.