Prefill community in post form from referrer

[kgilles]

In the post-creation form users have to select which community the post should be created in. We want to make things slightly easier for users who came to the form from a given community page, and prefill the community selector in these cases.

Depends On
#148

Criteria
When a user navigates to the post-creation page (/p) from a community page or post, prefill the community selector in the form with the community from which the user just came from