Mobile support

[benjaminkomen]

I'm writing this from the iOS GitHub app, I bet lots of people use it.

Currently stacked PRs are shown as regular PRs on mobile. I understand this is not the first priority but it would be helpful to at least have some indication on them on mobile to distinguish them from regular PRs. It would prevent accidental merging of a mid-stack PR.

[aliaksei135]

This is the same for Android. Additionally it is not possible to squash and merge from Android as the app as "merge must be done using the stack merge api", which I assume isn't supported on mobile

[FleetAdmiralJakob]

I'm writing this for the GitHub Android App aswell, but I bet that there is the same error on iOS aswell. Would be really nice to see the details of a stack in the GitHub mobile app aswell, the fact that this PR is part of a stack and being able to merge the stacks. I hate the "This pull request is part of a stack and must be merged sequentially using the stack merge API." error.