[WIP] Comments in sidebar (#935)

[artkravchenko]

Closes #935.

• /post/:id/comments
  The feed of comments in a separate page;
• /post/:id?route=comments
  Standard post page with the feed of comments in the sidebar;
• ?route=comments&post_id=
  The feed of comments to the post with given post_id in the sidebar, no post's content displayed;
• ?route=comments&post_id=&include_post
  The same as the item specified above but with post's content is visible;
• ?route=comments&comment_id=
  The separate thread of the comment with given comment_id in the sidebar;

[coveralls]

Coverage decreased (-0.08%) to 58.912% when pulling 663f359 on issue/935 into d738fa9 on stable.

[coveralls]

Coverage decreased (-0.06%) to 58.783% when pulling 9ae3c42 on issue/935 into 0773691 on stable.

[coveralls]

Coverage decreased (-0.2%) to 58.188% when pulling 5052684 on issue/935 into 233a973 on stable.