Issue #362 - Simple comments - Changes based on feedback recieved from #mediagoblin

Issue #362 - Simple comments - Changes based on feedback recieved from #mediagoblin

*   `db.models` - Removed `MediaEntry.get_comments()` and replaced it with a helper
    which just returns a cursor for the comments query
*   `media.html` - Added `{% set comment_author = comment.author() %}`
*   `user_pages.views` - media_home() now passes `MediaEntry.get_comments()`
    directly to `Pagination`, handles pagination for comments.
    *   Added `MEDIA_COMMENTS_PER_PAGE` to define the number of comments per page
        in the `media_home()` view.