Publish execution reports from pull requests

[Ryan McNally]

Currently we publish test reports and mutation testing results to the github pages branch, but this only from the main thread.

It'd be nice if we also published from pull request branches.

This should be fairly straightforward:

• enable the publish tasks for non-main branches
• Update the content ingestion script to put the reports somewhere appropriate
• Work out how to purge reports from closed pull requests - I'm assuming there's an actions hook for that.