[WIP] Followers

This is a feature branch for following people. Once finished, it will include:

* The ability to follow and unfollow people
* Automatic email notifications for followers when posting a new listing
* A "people you follow" grid in the profile page
* For the admin, the ability to turn this feature off
In this particular case, the tag name is used for added specificity so that `@grey-button .content` doesn't override the `display:none`. (In the other cases there's no need.) 

Is `!important` preferred?