Modify staticpage backend API to consume multiple languages

Implement the fallback mechanism on backend side
Implement external rest based staticpage backend
Render navigation using external rest based API
Adhocracy-service: Change API to get single external static page
Move all_languages and friends to i18n

and change the staticpage client-side to skip language choice and fall
back to the all_languages function.
adhocracy-service: Allow to specify base path in staticpages API
Better error handling for non-existing static pages
Allow hierarchical static pages

This now works with all 3 backends, but is mostly useful for kotti based
main navigation.
Remove static page sidebar div within main_content
Add second permission level for static page views

This adds a permission `static.show_private` which can be requested by
static page backends by setting the static page `private` attribute
`True`.
Allow to mark external static pages as private

Conflicts:
	src/adhocracy/lib/staticpage.py
Fix current external navigation item
Allow to split mediacenter and staticpages services

This allows to use separate mediacenter and staticpages API endpoints by
defining the more special configuration options:

* adhocracy_service.mediacenter.rest_api_address
* adhocracy_service.mediacenter.rest_api_token
* adhocracy_service.staticpages.rest_api_address
* adhocracy_service.staticpages.rest_api_token
extended KottStaticPages backend

Additional data fields: : nav, sidebar, description, css_classes (to
allow side specific customization).
change template for static pages

-  use sidebar and full_teaser
-  wrap everything in a static_wrapper div
   and apply css_classes there
Remove inner template blocks which also exists in inherited template

For some reason the content in the inner blocks isn't shown if these
blocks also exist in the inherited template (in this case
template.html).
Optionally allow unverified staticpages / mediacenter SSL API calls
extend kotti static pages backend with redirect_url data
Adapt CHANGES
Deterministic staticpage query strings by using ordereddict