UI for making Github pull requests, seeing status

[jonnor]

Requires being able to work on non-master branch (#527). Should then be able to mark it as a pull request, and write initial description. It should also be possible to see the status: merged/closed, new comments. And also the CI status (#526).
Seeing and making comments etc can happen on Github web interface, at least initially.

A possible stopgap would be to have easily available link to Github project/branch, because Github shows 'recently created branches' and allows two-click creation of PR from there.