[Fix bug 842306] Initial implementation of comments in events

* Add EventComment model and EventCommentForm
* Implement comment in events/views.py
* Add delete event comment to URLs
* Add delete comment permissions
* Fixes in CSS
* Send email to event owner on comment
* Add settings for email on comment
* Add tests
Merge pull request #363 from johngian/842306

[Fix bug 842306] Initial implementation of comments in events