Revert "admin/util: Simplify process_modelformset() and avoid dual-instancing"

This reverts commit a8d9c063c.
This partially reverts commit 41b78a4d.
Partial pick: Cleanup: removed commented code

Pick message: Partially picked because we seem not to have the code
    that gets altered in the other changes in the original commit.

Conflicts:
	pootle/apps/pootle_store/views.py
Tests: don't use mutable type as a default argument
styleguide: Use an import that's not deprecated in examples
Accounts: Use get_user_model() instead of User
Cleanup: Remove unused import
Added missing imports
Fixed variable reference
Accounts: Work around cyclic validation errors
Voting: Drop voting

It is largely unused. django-voting is also incompatible with custom user
models.
Changelog: Fixed wording
Changelog: Mention voting being dropped
Changelog: Update expected release date
staticpages: Use lazy User foreign key
requirements: Update some requirements

We were affected by a nasty South bug in legalpage migrations.
https://stackoverflow.com/questions/20446724
Requirements: Update Tastypie requirement

Necessary to avoid some warnings, to get a lot of bugfixes and to
have a recommended version.
Travis: Disable pytest
requirements: Update Django requirements
requirements: Update various requirements versions

Also drop exact versions for travis as we will always want the latest ones.
requirements: Update Raven optional dependency to 5.x