sass: refactor and make more themeable

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