New in-app-menu and Template cleanup

Created by: fonfon

This pull request consists of #28 and the new submenu from https://github.com/fonfon/Plinth/pull/1. If you want to merge #28 first/separately I can make a new pull request. This is a cleanup of the submenu and template-structure, removes some manual python template rendering and includes these changes:

• removed 'in-app-menu' (subsubmenu) from bottom left (strangely called 'sidebar_right'); no more manual rendering in python for the menu
• the subsubmenu is now on the top of the content, uses http://getbootstrap.com/components/#nav-tabs and has highlighting functionality; see screenshot at the end of this text
• renamed and partly restructured template blocks:
  • removed sidebar_right and sidebar_left (just use sidebar)
  • renamed all _block template blocks to normal variable names ('content', 'title', ..)
  • renamed main_block to content (no strong opinion here, I'm perfectly happy with leaving it main too)
• the <h2>{{ title }}</h2> can/should be defined directly in the template now
• xmpp: moved sidebar-content to page and added a 'launch web client' button
• made tor 'enable hidden service' an inline (one-line) form
• 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

@SunilMohanAdapa reviewed parts of pull request #28 already.

Here's a slightly outdated screenshot of what the new subsubmenu looks like (it's the 'Configure XMPP Server' and 'Register XMPP Account'):