Update old and/or redirected links

I have already fixed many of these, but a few remain:

* [ ] `fiji.sc` (check case by case)
* [ ] `fiji.sc/downloads` :arrow_right: `downloads.imagej.net/fiji`
* [ ] `fiji.sc/git` / `git.imagej.net` / `fiji.sc/gitweb.cgi` :arrow_right: `{% include github ... %}`
* [ ] `forum.imagej.net` :arrow_right: `forum.image.sc`
* [x] `maven.imagej.net` :arrow_right: `maven.scijava.org`
* [ ] `fiji.sc/javadoc` / `javadoc.imagej.net` :arrow_right: `javadoc.scijava.org`
* [ ] `jenkins.imagej.net` :arrow_right: ???
* and in general look for all links to imagej.net, fiji.sc, scijava.org, scif.io, etc., and ensure they are up-to-date

See also #66.

Search the repository for links to imagej.net, fiji.sc, scijava.org, and scif.io, then review the remaining checklist items and issue #66 for context. Update outdated links to the targets listed, determine the replacement for jenkins.imagej.net, and finish when the relevant links are current.

Written by the indexing model from the issue text.