Update old and/or redirected links

[ctrueden]

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

• fiji.sc (check case by case)
• fiji.sc/downloads ➡️ downloads.imagej.net/fiji
• fiji.sc/git / git.imagej.net / fiji.sc/gitweb.cgi ➡️ {% include github ... %}
• forum.imagej.net ➡️ forum.image.sc
• maven.imagej.net ➡️ maven.scijava.org
• fiji.sc/javadoc / javadoc.imagej.net ➡️ javadoc.scijava.org
• jenkins.imagej.net ➡️ ???
• 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.