Onboard production engineers on Gitter infrastructure

The time has come for the production engineering team to start operating on Gitter's infrastructure. There is currently only one person on it (myself) and the bus factor is too low (close to 1 since @andrewn is confident with good part of the codebase).

To do this I'll need to write more documentation and then use it to train the rest of the team. As an exercise I can create issues for things that need/I'd like to be done and supervise other production engineers' work.

Ideally Gitter will be an integral part of our daily job and it'll become part of our on-call, replacing the existing one.

/cc @mydigitalself @pcarranza