Bootstrap3fixes

Created by: fonfon

A bunch of small UI-related changes from #27 (which I'll re-open to keep the menu changes separated):

• 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