Add support for choosing global stylesheets in includes `css` and `js`
Only display GitHub-project specific fileds if actually used

So if I re-use this for a site that does not have a GitHub project
associated with it, those extra links and buttons won't be included.
Separete off "GitHub Project" specifics from Slate CSS
Move slate-specific images to subdirectory
Tweaks to Slate theme and structure in Jekyll project
Add `flink`

See https://github.com/IQAndreas/flink