Templatefixes

This pull request is based on bootstrap3 #23. The main changes to #23 are:
- refactoring templates: renamed and restructured blocks, deleted things that weren't used (anymore)
- moved the 'in-app' menu from the bottom left to the top of the content

The new menu on top of the main content would look like that:

![newnav](https://cloud.githubusercontent.com/assets/1867309/5125791/0ee9161e-70c8-11e4-9dca-3d27489e61af.png)
 
In my opinion there were some issues with the old menu:
- it could be overlooked easily, bottom left is not a prominent place to look for a menu
- it would go further down the more menu-items (i.e. apps) we add
- there was no highlighting of active links
- it wasn't always treated as a menu, i.e. on sys/users/ if you clicked 'add user' the menu disappeared

I'm happy to discuss/see other menu proposals, I played around a bit and this was the concept I liked most: one 'main' menu on top, the 'submenu' on the left side, and the 'subsubmenu' on top of the content. For additional information (but no navigation) there's still the sidebar below the submenu.

Other changes:
- I removed the "Chat" link from the app menu. Similar to owncloud there's the link in the xmpp sidebar now. With the dashboard (#20) we'd have links to apps collected on one place.
- the `<h2>{{ title }}</h2>` isn't added automatically to each page anymore. On pages where we have a subsubmenu this would be too much, i.e. submenu: "XMPP", subsubmenu: "Configure XMPP", and then the title "XMPP" on top of the content.
- don't read/render sidebar-content with `render_to_string` in python, but use django templates instead. Only exception: the 'help' detail pages.