HTML mode content changes now trigger change callbacks.

Fixes [this issue](https://github.com/wordpress-mobile/WordPress-iOS-Editor/issues/437).

I could only test this in the demo app, and not integrated into WPiOS due to [this PR](https://github.com/wordpress-mobile/WordPress-iOS/pull/2943) not being approved yet.

I'd suggest to not merge this code until we can properly test the integration, but I'm still posting the PR for code review & testing in the demo app.

/cc @bummytime