Github workflow to test Galaxy Lab changes

[neoformit]

When someone makes a PR with changes to a lab, a comment will be posted in the PR providing links to preview the lab page(s) for which changes have been made. This allows both the actor and reviewer to view the rendered webpage before merging. This is especially useful for ensuring there are no fatal errors before merging.

The github-actions comment will also test HTTP status for each version of the Lab page, and show the result in the comment. The test is re-run and the comment updated with every push, so you can see if a commit breaks or fixes a Lab page.

Test success following changes to spoc Lab:

Test failure following changes to spoc Lab (merging this would break the production spoc Lab):

[neoformit]

Also - it is apparently possible to just use the default GITHUB_TOKEN (that gets injected into all actions) for my purpose here, but it didn't work when I tested it. I will try testing it again.

The GitHub docs indicate that this is possible, but the GITHUB_TOKEN in my workflow run lacks sufficient permissions to post a comment to the PR.

There is also another approach that uses artefacts to post the PR comments in a separate workflow - I could have a look into that.

I have tested this out and it now works without the need for a dedicated github token - this is now ready to merge if we can resolve #201 (comment) @bebatut