Merge branch 'ben.boeckel/gitlab-ce-fixup-links-in-generic-docs' into 'master'

Fix relative links in other markup formats

_Originally opened at !1845 by @ben.boeckel._

- - -

The RelativeLinkFilter was not applied to other document formats, e.g.,
reStructuredText, so links from the Files view or the Project view did
not work.

Fixes #3533.

See merge request !2798