Browsable API cleanups.

Closes #1719.

* indentation in the CSS files made consistent to 2 spaces
* spacing between lines fixed
* space added before the curly brace where needed
* `<div id="push>` removed
* `<footer>` added, `{% block footer %}` moved into `.wrapper`, sticky footer realized with `padding-bottom` to `#content`
* two waste closing `</div>` tags in the end of base.html removed
* indentation of the complete file base.html rearranged, now it should be better readable (imho)
* color codes in CSS made consistent, always using capital letters and being 6 chars long
* `.navbar` is now fixed, so it looks same like the project homepage
* small make-up corrections in bootstrap-tweaks.css