Split themes off into individual .css files

config.example.js now lists which themes are available for use by the client.

TODO: Auto-generate 'thumbnails' for the themes in the settings applet.
Auto-generate 'thumbnail' previews of themes in settings applet
Misc fixes

Correctly define link and remove superflous `var set_theme`
Move themes from client/assets/src/themes to client/assets/themes
Load themes as soon as possible to avoid FoUC

Also move theme configuration slightly to avoid sever-side caching bug
Merge branch 'themes' of https://github.com/M2Ys4U/KiwiIRC into M2Ys4U-themes

Conflicts:
	client/assets/css/style.css
	client/src/applets/settings.js
	client/src/index.html.tmpl
	server/httphandler.js
Switching stylesheets - specify node type in query selector
CSS images moved to relevant theme folders
Re-jigging UI layout after a theme change
Fix theme switching bug
Separated httphandler and settings generation