Feature/4646 collapse post button

Adds an icon in the upper right corner of each opened post, visible on mouseover. Clicking the icon will collapse the post.

No tests for this feature so far included!
I tried several hours to figure jasmine out ... but gave up at some point. If anyone has a hint how to instantiate a fully initialized post (where the content_view.js has been run), I will gladly implement tests for collapsing and expanding posts.
Our current `And I debug` step launching pry should be enough. Pressing Ctrl+D to continue is hardly harder than pressing any key.