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
The whole `sidebar` needs an overhaul. sidebar_left isn't used, sidebar_right is on the left side and is easy to not be displayed as it was the case here.
The overhaul was part of #27 already, to be re-opened soon;