Add allow_removal option to our FormBuilder#upload helper

This will add a remove_xxxx field to the form (using a rails style checkbox) and it works because if xxxx is a carrierwave mounted thing, then remove_xxxx is a magic carrierwave attribute to blank it out.
Allow removal of logos from topical events

If the topical event has a logo, then render a checkbox to allow removing it.
Allow preview of rendered topic & topical event descriptions

They are govspeak after all, it seems a shame not to.
Merge pull request #544 from alphagov/allow-removal-of-topical-event-logo

Allow removal of topical event logo

https://www.pivotaltracker.com/story/show/49117861