Custom showdown extensions should be linted

ref #4243

- Removing /lib/ from the list of things which aren't linted, as this is
  all custom code
- Fixing up the two files to pass linting, erring on the side of caution
  with disabling regexp rules vs fixing regexes!
Same as above.  Sorry if you've tested it out, just want to make sure we're not introducing a subtle bug.