Move group wiki specific code into EE

Summary

The new group wikis will be available in GitLab Starter and up, following the decision at &240 (comment 334075256).

Improvements

We'll have to change some of the already implemented work:

#207864 (closed): Move GroupWiki → !30857 (merged)
#207872 (closed): Move group wiki policies on GroupPolicy

Edited Apr 30, 2020 by Markus Koller