Fixed links to other threads and corrected layout.

$ Fixed >>links to other threads within the same board.
$ Fixed an issue with thumbnails height in the board view.
Introduced flexbox instead of floats where applicable.

Website was previously using floats due to the support concerns in the older browser releases. At this point all current browser releases support flexbox so I decided to switch to it.

+ Replaced floats with flexbox in the board view, index view, posts and pagination.
  Flexbox was also introduced in the board view links which makes the website render fine without JavaScript. At this point JS is there only to make certain elements equal in size. That makes rows of threads easier to follow while reading.
* Filters moved from template board.html to board_filters.html.
Fixed board appearance in Chromium.