Sass: refactor and make more themeable

  - sass: put partials in lib/{.,mixins}
  - document and make themeable
  - add example themes: 'flatly' and 'darkly' from bootswatch

Squashed commit of the following:

commit 5af8d680ff4cdf4ff98beae6e8e156b6045a206b
Author: Max F. Albrecht <1@178.is>
Date:   Sat Dec 13 19:11:21 2014 +0100
UI fixes

Squashed commit of the following:

commit 2f98c87bc7511da78a10723d971e08e0d98632ef
Author: Max F. Albrecht <1@178.is>
Date:   Sun Dec 14 01:55:02 2014 +0100

    sass: executions-table: tone down the background colors, add top border

commit e758d6e6872826b49417ba5cea41064a0e9eb092
Author: Max F. Albrecht <1@178.is>
Date:   Sun Dec 14 03:17:41 2014 +0100

    ui: navbar: show version next to brand IF it's a pre-release

commit 9c33d7b1816e4e447bb74baaf0b5a654f5b9c9f3
Author: Max F. Albrecht <1@178.is>
Date:   Sun Dec 14 03:16:54 2014 +0100

    haml: add partial 'semantic_version'

commit a8230d9dcbe468f01ed408ed239e59514ef935fb
Author: Max F. Albrecht <1@178.is>
Date:   Sun Dec 14 03:16:01 2014 +0100

    rails env: Set Haml output to ugly everywhere

commit 63217337f0b3976fb040c873bb1aabaac959c2e8
Author: Max F. Albrecht <1@178.is>
Date:   Sun Dec 14 01:53:51 2014 +0100

    sass: make colors_and_states themeable

commit b27617328efedbbfe086cbda3aa6745fd64bdf96
Author: Max F. Albrecht <1@178.is>
Date:   Sun Dec 14 01:53:36 2014 +0100

    sass: small visual fixes

commit 8d027361f0ff1376583726a0b46b001e0df20481
Author: Max F. Albrecht <1@178.is>
Date:   Sun Dec 14 01:51:39 2014 +0100

    views: workspace/executions: remove tr.spacer from table

commit 4714a4dd78177a2c69c834465bb2757f2d4e267c
Author: Max F. Albrecht <1@178.is>
Date:   Sun Dec 14 01:51:03 2014 +0100

    haml: remove whitespace from inside .label
add static /public/ui-demo
start cider theme (based on bootstrap variables)
#tmp