Previously, when the wiki page format was anything other than `markdown`
or `asciidoc` the formatted content would be returned though a Gitaly
call. Gitaly in turn would delegate formatting to the gitlab-gollum-lib
gem, which in turn would delegate that to various gems (like RDoc for
`rdoc`) and then apply some very liberal sanitization.
It was too liberal!
This change brings our wiki content formatting in line with how we
format other markdown at GitLab, so we have a SSOT for sanitization.
https://gitlab.com/gitlab-org/gitlab/issues/30540
Because the post-processing of the rendered README is dependent on the
context (i.e. the current user), do the post-processing when the
README is being displayed.
- Apply the RelativeLinkFilter filter to other formats, e.g.,
reStructuredText so links from the Files view or the Project view work
- Remove the AsciidocPipeline pipeline
Fixes#3533.
