Many css related fixes.

* remove units from 0
* remove duplicate properties
* merge duplicate selectors
* remove incompatible property `display: table-cell`
* add missing vendor prefixed properties and move the unprefixed property after the vendor ones
* use the shorthand properties when possible and be more explicit with the background-color and list-style-type property
* use the (shortened) hex colors when possible and be consistent with the case used
* move overrides after bootstrap
* move inline style to style.css
* use the bulletproof syntax for fonts
* use the existent Bootstrap class clearfix and clean up unneeded selectors
Trim trailing spaces and use 2 spaces consistently for indentation.
Fix validation errors.

All validation errors are fixed except for the `http-equiv="X-UA-Compatible"` meta tag which is known to not be HTML5 valid but is needed in order to force usage of the latest IE.
Remove stray space after `alt=`.
index.html: remove unneeded class.

It's inherited from `ul`.
Fix links.

* use the https links to GitHub for consistency
* fix kramdown links target
Use the same dimensions as in the css for .avatar.