Fix wiki linking behavior for markdown wiki pages

Related to #18019 (closed)

• As per the documentation in !4372 (merged)

TODO

• !4432 (merged) Have wiki linking behave as per the documentation
  • Move WikiLinkFilter specs to the pipeline level
  • Verify current behavior on wiki show page
  • Fix current behavior on wiki show page
  • Verify current behaviour on wiki preview
  • Fix current behaviour on wiki preview
    • Rewrite all links and get preview links working
    • Make sure all links are on-par with the wiki show page
    • TDD WikiLinkFilter and get it working
    • Hook WikiLinkFilter up
  • Fix tests
    • Fix markdown_spec
    • Fix wiki spinach feature
    • Wait for build to pass
    • Make sure all wiki-related pages are working as expected (history, all pages, etc.)
  • Test in different ruby versions
  • GitLab instances hosted on a relative URL
  • Non-markdown rendering formats?
    • RDoc
    • ASCIIDoc
    • Create issues to fix things for RDoc and ASCIIDoc
  • Gauge performance impact
  • Refactor
  • Re-organize commits
  • Make sure build passes
  • Respond to @rymai's comments
    • class WikiLinkFilter < HTML::Pipeline::Filter
    • blank line after guard clause
    • keyword arguments for wiki and slug
    • invert the condition
    • inline user in spec
    • Make sure spec names are not out of date
    • Comment for each rewrite rule
  • Add CHANGELOG entry
  • Reorganize commits
  • Make sure build passes
  • Wait for merge