Enforce passing context to super in class constructor

[miklschmidt]

This is especially important for relay classes.

[kastermester]

We need to figure out if this always applies or not. I suspect something else was the actual bug here (but I'm not sure).

At the very least however, we should create a rule that enforces this when referencing this.context inside the react class.

[miklschmidt]

Yes, i think it actually wasn't the source of the problem. There's probably documentation somewhere that explains when it's necessary.