Remote staging branches

For example, to deploy #4212 (before it was merged), staging.yaml would have
```yaml
trunk: master
name: autostaging
branches:
  ...
  - lwyszomi:ilsgateway-integration # <----------- REMOTE BRANCH
submodules:
  ...
```
The branch name `lwyszomi:ilsgateway-integration` can be copied directly from the top of the github PR page where it says
"@user wants to merge 2 commits into `dimagi:master` from `lwyszomi:ilsgateway-integration`".
This is really handy for testing changes to rebuildstaging.py