moved 'in-app menu' from the sidebar to the top of the content

@SunilMohanAdapa: I made a separate pull request here to keep these changes separate from the other bootstrap3 changes. It's a slightly cleaned up version of what https://github.com/freedombox/Plinth/pull/27 included already.
I'll make a pull request to freedombox/Plinth when the bootstrap3 changes are merged.

Changes:
- moved submenu to the top of the content and added highlighting functionality
- renamed and partly restructured template blocks:
  - removed sidebar_right and sidebar_left (just use sidebar)
  - renamed all `_block` template blocks to 'normal' variable names
  - 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