Changelog for upcoming release 12.0

Every project should have a changelog.
It's very nice for library users, who will upgrade from previous versions.

I've skimmed through [243 commits since last release 11.12](https://github.com/mariocesar/sorl-thumbnail/compare/v11.12...588837) and summarized all significant changes.
Excuse me if I've missed somebody's contributions, it wasn't intentional.

The order is chronological by commit date, except for first 2 entries.