We should track notification engagement

[eemeli]

As we now have a bunch of notifications generated by Pontoon, it might be useful to track if the links on them get clicked. This would allow us to get some data on their utility, and maybe help us weed out or refactor notifications that aren't as useful as we might have hoped.

To ensure that no personal data is exposed from any such tracking, we'll need to build our own backend for this.