Defer loading of certain extra JS/CSS and only load when necessary

jQuery UI is only needed on 'leden' pages so it has been moved to there.
CSS of jQuery UI and Nivo Slider is loaded inside a `DOMContentLoaded` event
handler.
JS of jQuery UI and Nivo Slider has got the 'defer' attribute, which means it
will not block page rendering.
Move CSS defer script from base.html to to bare.html
Merge branch 'master' into defer-css-js

Conflicts:
	kn/base/templates/base/bare.html
Merge pull request #217 from aykevl93/defer-css-js

Laadt bepaalde JS/CSS nadat de rest van de pagina geladen is (2)