Drop django-registration app from Pootle codebase

Seems that the code was not on Pootle codebase because it was necessary to
localize it, but perhaps because this way was easier to release Pootle.

The new registration app doesn't save the RegistrationForm, so we can't
rely on it for saving the agreements accepted when the user registered.

Use pinned version for the package in requirements to avoid ugly surprises.

Fixes bug #2579
Drop django-contact-form from Pootle codebase

Since this app was kept for localization a custom app that extends
django-contact-form was put in place just to keep the form localized.

Aligned with Evernote's approach as much as possible while keeping it simple.

Use pinned version for the package in requirements to avoid ugly surprises.

Fixes bug #3139
Profiles: Apply PR to get rid of function-based generic views

The PR is in https://bitbucket.org/ubernostrum/django-profiles/pull-request/6/

There is another PR with a slightly different approach, but when and if
django-profiles finally supports class-based generic views the code will be
quite similar to any of these two PRs.

Whitespace changes are not merged to make it easier to see the real changes.

This commit and the previous two ones fix bug #2517
Bump required Django version to 1.5.5
Update Travis config to test latest Django releases