New app-menu and Template cleanup

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Here's a slightly outdated screenshot of what the new subsubmenu looks like (it's the 'Configure XMPP Server' and 'Register XMPP Account'):
![newnav](https://cloud.githubusercontent.com/assets/1867309/5125791/0ee9161e-70c8-11e4-9dca-3d27489e61af.png)