Mass cleaning of kfet' authentication machinery

AccountBackend
- Should now work if used in AUTHENTICATION_BACKENDS settings.
- It does not retieve itself the password, as it should not be used
this way.

GenericBackend
- Delete useless 'username' arg of its 'authenticate()' method.
- Now delete the token in DB.

TemporaryAuthMiddleware
- New name of the middleware is more meaningful.
- Is now responsible to retrieve the password from the request, instead
of the AccountBackend.

GenericTeamToken model
- Add a manager' method to create token, avoiding possible error due to
unicity constraint.

GenericLoginView (authentication with the kfet generic user)
- Replace obscure system with a 100% HTTP handling.
- See comments for more information.

Misc
- More docstrings!
- More tests!
- Add some i18n.
- Add kfet/confirm_form.html template:
    Ask user to confirm sth via a form (which will send a POST request).
    Context variables:
        * title: the page title
        * confirm_url: action attribute for <form>
        * text: displayed confirmation text
- kfet.js : Add functions allowing to emit POST request from <a> tag.
- Non-link nav items from kfet navbar also get a 'title'.
- A utility has been found for the 'sunglasses' glyphicon!

kfet/auth/signals.py

@@ -0,0 +1,40 @@
+from django.contrib import messages
+from django.contrib.auth.signals import user_logged_in
+from django.core.urlresolvers import reverse
+from django.dispatch import receiver
+from django.utils.safestring import mark_safe
+from django.utils.translation import ugettext as _
+
+from .utils import get_kfet_generic_user
+
+
+@receiver(user_logged_in)
+def suggest_auth_generic(sender, request, user, **kwargs):
+    """
+    Suggest logged in user to continue as the kfet generic user.
+
+    Message is only added if the following conditions are met:
+    - the next page (where user is going to be redirected due to successful
+        authentication) is related to kfet, i.e. 'k-fet' is in its url.
+    - logged in user is a kfet staff member (except the generic user).
+    """
+    # Filter against the next page.
+    if not(hasattr(request, 'GET') and 'next' in request.GET):
+        return
+
+    next_page = request.GET['next']
+    generic_url = reverse('kfet.login.generic')
+
+    if not('k-fet' in next_page and not next_page.startswith(generic_url)):
+        return
+
+    # Filter against the logged in user.
+    if not(user.has_perm('kfet.is_team') and user != get_kfet_generic_user()):
+        return
+
+    # Seems legit to add message.
+    text = _("K-Fêt — Ouvrir une session partagée ?")
+    messages.info(request, mark_safe(
+        '<a href="#" data-url="{}" onclick="submit_url(this)">{}</a>'
+        .format(generic_url, text)
+    ))