[2025] Public external link checker

[pcraig3]

The NOFO Builder has 2 link-related features:

1. Check all external links to find 404s
2. Check all internal links to find broken ones

However, to use either of these, you need to be an official NOFO Builder user who has logged in and uploaded a document.

We have been seeing some OpDivs send us their doc, then someone uploads it and finds lots of broken links, then sends it back to the OpDiv.

We could save time and energy if we make the link-checking tool public and directed our OpDiv partners to try it out.

More discussion is needed about how precisely this would work, since NOFO content should not be public before it is published (so we need to hide imported NOFOs). But we should be able to give people a readout they can use to check their NOFOs' links are not broken.