Ensure there are no broken links, and automate the check

The script `_bin/broken-links.sh` prints out links that it detects as broken. But it needs updating to handle additional cases:

* `/ij/*` – proxied mirror.imagej.net content – needs update to serve from `/ij` and redirect old links from the root where feasible (e.g. `/macros`)
* Other repos in this org: `/presentations`, `/workshops`, `/tutorials`, `/list-of-update-sites`, others?

List of known observed weirdness so far:
* `BigDataServer: INFO__` are linkish, but are surrounded in backticks. These should be fixed, and other instances of backtick-mangling should be checked for.
* Some MediaWiki-style links got escaped with backslashes—`\[link title\]`—I fixed many of them but would be good to double check there aren't any remaining.
* `(Category_Segmentation)` (and similar) links—and `_bin/broken-links.sh` does not find them.

Once we have a robust dead link checker, we also need to hook it up to an action to check when links break.

See also #55, #63 (IJ1 page renames), #66