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