WIP: Get FastLane working with iOS Gitter builds
Now that Android is working, let's get iOS builds working using FastLane.
Explicitly left out of scope here is trying to merge them into one build pipeline. I think that would be cool to do, but would add a layer of complexity not needed initially. In practice will try to keep them as similar as possible so that it's easy later.
We're going to need a mac to build - pinged on our internal issue for hosting Macs (and gitlab-com/gl-infra/infrastructure#5720), but if that isn't available we'll need to get a host on MacStadium or MacInCloud.