Update bootstrap and refactor theme templates

- Do not work against the system (mixins)
- Use correct block/inline level elements
Bugfix: elements used as block elements should BE block elements
Bugfix: write correct HTML syntax

First try to produce correct semantics and then - ONLY THEN - apply
styles to the html template.
Reenable all included less files
Remove important statements from application.less

If you need one !important statement than you are probably doing it
wrong...28(!) are definetely too much and the css should be refactored