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
haml: remove whitespace from inside .label
views: workspace/executions: remove tr.spacer from table
sass: small visual fixes
sass: make colors_and_states themeable
rails env: Set Haml output to ugly everywhere
haml: add partial 'semantic_version'
ui: navbar: show version next to brand IF it's a pre-release
sass: executions-table: tone down the background colors, add top border