Use a drop-down menu for navigation instead of tabs

Fixes evernote/pootle#76.

Pick notes: Fixed several conflicts due to cleanups we did, code divergence,
    code we kept, and code we didn't pick. This requires cleanups to
    properly show the dropdown (it is always hidden). Also recovered some
    styling for the breadcrumbs that we didn't pick in the past.

Conflicts:
	pootle/apps/pootle_project/views.py
	pootle/apps/pootle_translationproject/views.py
	pootle/static/css/style.css
	pootle/templates/editor/main.html
	pootle/templates/languages/_tabs.html
	pootle/templates/projects/_tabs.html
	pootle/templates/translation_projects/_tabs.html
	pootle/templates/translation_projects/base.html
Followup: Do not hide the navigation dropdown

Like we did for the resources one in 001c0cba.