Remember the last language users switched to

This allows to:
  - Bring users to the overview page of the last language they
    switched to.
  - When clicking on "Go back to overview", always redirect to a overview
    page of the current language being translated.
Avoid redirect loops by always setting a language code

When clearing both language and project selections this will ensure
'projects' is set as a language and therefore will redirect to /projects/.
As a safety measure, also check for the empty string
Always remember the last visited language

Pick note: Just fixed the conflicts.

Conflicts:
	pootle/apps/pootle_language/views.py
	pootle/apps/pootle_project/views.py
Partial pick: Switch to Python's own json module

Pick note: The rest of the changes were already picked.

Conflicts:
	pootle/apps/pootle_translationproject/views.py
	pootle/core/forms.py