Previous context display

When a string becomes fuzzy because of a simple context change, the translator can see there is no source change, but the reason "needs review" is now ticked is not apparent to them - they have to guess it's a context change (worst case, they think it's a bug and directly accept the fuzzy...) and download / look at the .po file where previous context is indeed present.

I think we should display it too (a red/green diff view between prev and current context is probably not mandatory - just display previous context)