Email validation

This MR bring email validation to debexpo.

The workflow is similar to the password reset:

• Request the change from the profile page
• Open the token link sent to the new email
• Validate the change

However, django password reset helpers had to be adapted to an email update. In order to do so, two classes have been extended:

• EmailChangeConfirmView from django's PasswordResetConfirmView
• EmailChangeTokenGenerator from django's PasswordResetTokenGenerator

The changes ensure a token is invalidated when the email changes in DB and cannot be used with another email.

I've updated the testing env at https://debexpo.debian.net to run this branch. (Note: email are not sent on this env but stored in /home/lyknode/projects/debian/repo/lyknode-guest/debexpo/mbox)

Other than that, tests and translation have been updated as usual.

Closes #83 (closed).