Checking LMFDB for broken external links

[jenpaulhus]

Today someone noted on a bug report form that we had a broken external link for a paper referenced in a knowl. I fixed the particular example (an msri.org file that is now located at slmath.org) but it would be nice to have a system that occasionally checks the lmfdb for broken external links.

[edgarcosta]

We already have a tool for that (except, it never went through the knowls), but at some point, we decided to disable them from the GitHub workflow tests, as many of the external links were not reliable, so we had too many false test failures because of these.

https://github.com/LMFDB/lmfdb/blob/main/lmfdb/tests/__init__.py#L60-L75

[AndrewVSutherland]

I don't think we want to do this as part of our GitHub/CI workflow, but I do think it would be useful to have a test we could run on demand that would also cover knowls.