Show latest comments in sidebar.

- CommentRepository.php >> Created getLatestComments function to fetch latest comments from the database (takes parameter $limit for # of comments to fetch), and return an array.
- PageController.php >> Fetches # of comments (limit) from config.yml, and calls getLatestComments in CommentRepository.php, then passes return to the view.
- config.yml >> Added number of comments to fetch for the sidebar "widget" as parameter blogger_blog.comments.latest_comment_limit.
- sidebar.html.twig >> View for the sidebar: added new section that shows latest comments.

Signed-off-by: Parth Sarin <parthsarin@gmail.com>