This moves all URL references to Trac Wiki to their corresponding
GitLab counterparts.

This substitution is classified as follows:

1. Automated substitution using sed with Ben's mapping rule [1]
    Old: ghc.haskell.org/trac/ghc/wiki/XxxYyy...
    New: gitlab.haskell.org/ghc/ghc/wikis/xxx-yyy...

2. Manual substitution for URLs containing `#` index
    Old: ghc.haskell.org/trac/ghc/wiki/XxxYyy...#Zzz
    New: gitlab.haskell.org/ghc/ghc/wikis/xxx-yyy...#zzz

3. Manual substitution for strings starting with `Commentary`
    Old: Commentary/XxxYyy...
    New: commentary/xxx-yyy...

See also !539

[1]: https://gitlab.haskell.org/bgamari/gitlab-migration/blob/master/wiki-mapping.json