Fix bug 856121: Use Django's sensitive-data controls.

This basically only applies the 'sensitive_post_parameters' decorator
to a few views in the users app; we probably don't even need them
there, but since that app has some forms that can handle passwords,
better safe than sorry.
Merge pull request #1051 from ubernostrum/sensitive-data-856121

Fix bug 856121: Use Django's sensitive-data controls.