Continued fixups

[timwoj]

This PR should fix all of the remaining warnings from debug.log from the last few days on the live site.

• The metron-bro-plugin-kafka package uses anchor links in its readme to link to different places in the readme. The markdown canonifier throws a deprecation warning in debug log because those links aren't handled correctly. Add the HeadingPermalinkExtension to the canonifier to add the anchors to the output, and fix the handling of the anchor links to not result in the debug log.
• Fix the paths to the error template files (in the error template files). I don't know what this actually does for us. I should probably revisit those files at some point to see if we can make them actually useful.
• Fix the path to the favicon in the header meta tags so that we don't get a bunch of 404s in error.log.
• Switch to a different ReST markup parser that has newer releases, which fixes a bunch of debug.log warnings with the old one.
• Fix parsing of <img> tags in Markdown READMEs like we do for the []() format for so that we link to the right image files. I noticed this while looking at the JA4 package, which was missing its header image.

[timwoj]

Ignore that force-push. I accidentally pushed these to master before making the branch.

[ckreibich]

LGTM Tim — as usual I likely won't spot bugs in there. The whole thing is a good reminder to go simple in one's READMEs...

[ckreibich]

Maybe add a comment as to what this does?