Restructure help urls and views, add default template

Can now handle user set language and fall back to
default settings language
Make celery_haystack an optional app
Show error message when no FOI law can be found
Remove optional runtime requirements

- no more django secure
- no more celery email
- no more celery haystack
- remove doc dependencies
- remove too environment specific requirements
Define package install requirements
Improve foi attachment url methods
Add ThemeBase configuration mixin
Make dj_static's Cling optional in wsgi.py
Fix help_view if locale middleware is not installed
Add mock library to test requirements.txt
Fix test to work without celery_haystack signals
Update some button css and icons to glyphicons
Don't show section headings on index page if empty
Add small changes for Py2/3 support
Fix csv import with ugly switch for Python 2 and 3
Catch missing file objects on attachment
Remove preview URLs

Browsers support PDF previews now