Bootstrap3fixes

A bunch of small UI-related changes from #27 (which I'll re-open to keep the menu changes separated):
- reduce the size of the sub-navbar (on the top left)
- remove unused template `form.html`
- update texts of the apps 'users' and 'first_boot'
- again display the help text for first_boot (it was gone, probably unintentionally)
- remove the template-block `js_block`: use `app_js` or `page_js` instead
IMHO the old subnavbar looked cleaner, it was easier to read and find single items. But I don't have a strong opinion here, I'm fine with reverting this change.

What do you mean with the vertical bootstrap navbar? Bootstrap doesn't offer any explicit vertical navbar, there are only elements that you could use for that purpose, like we do (linked lists or the 'stacked nav pills' we are using).

I agree that we should be careful with adapting bootstrap. In this case - imho - it reduces mobile usability but increases readability, so I'm okay with both.