Events recent comments [JOINDIN-216]

Issue https://joindin.jira.com/browse/JOINDIN-216

Adds a new tab to the events main page that shows comments to all the talks of the event in reverse chronological order. Loading of the comments is done by an ajax call and it's paginated (currently loads 20 comments per call).

I noticed @lornajane changed the Component tag in Jira from "Original website" to "Responsive website" on this issue a few days ago, but I had already started working on this before that so I thought I'd send a PR anyway. I guess I'll work on this for the web2 version next.

I have also asked for feedback about the tab naming in the mailing list but have not gotten any replies.
https://groups.google.com/forum/#!topic/joindin-developers/deYa4gT_C3k