Used button mixin on all submit inputs
Moved article wrappers out of content.php

I've done this so that the article can be a wrapper for the comments.
Semantically comments will be better as child articles of the main content.
Improved Custom comment markup

- removed callback function and created walker class instead
- used article markup so that comments will be child articles of the main one
- used foundation columns and panel styling as well as sub-nav for meta links