Generalize Previous Versions logic?

[Eric-Arellano]

This would be like how #302 generalizes the translations features so that users only need to set some conf.py config to use translations. We would similarly make it easier to use versions without copying and pasting custom extension code.

However, I'm not even sure if this is viable because previous versions logic differs a little between projects. At worst, we may only be able to provide some helper functions.