Use django-browserid instead of social-auth

The django-browserid package is much more secure, and managed by the
Mozilla folks.
Strip tags when added

Fixes: #56
Clear the add tag form when the request is done

Fixes: #57
Allow adding multiple tags at once

Tags must be separated by commas or semicolons. The suggestion mechanism
does not take that into account for now.
The comments number is the thread emails count minus one

Fixes: #64
AUTH_PROFILE_MODULE is deprecated since Django 1.5
Reorganize tests
Don't pretend we can attach files when we can't

jQuery won't let us do that, see http://bugs.jquery.com/ticket/13306
The solution is to use the File Upload plugin:
http://plugins.jquery.com/blueimp-file-upload/
But it's a much bigger work (dedicated controller, temporary files that
need cleanup, etc) so it'll be in the next version.