GHC wiki fixup

[Tobias Dammers]

Hello all,

I am about to push a relatively large set of semi-automatic changes to
the GHC wiki on GitLab (https://gitlab.haskell.org/ghc/ghc/wikis).

This changeset is the result of re-running the original Trac import,
with a few bugfixes applied to it, and then cherry-picking the resulting
changes to the wiki onto the live wiki. Among other things, this fixes
the following issues:

- Mangled table layouts (GitLab's Markdown implementation does not
  support inline markup inside tables):
  https://gitlab.haskell.org/bgamari/gitlab-migration/issues/45
- Mangled code blocks (for some reason, the original import threw away
  significant whitespace inside syntax-highlighted code blocks):
  https://gitlab.haskell.org/bgamari/gitlab-migration/issues/37
- Some links to GHC source files pointing to
  gitlab.haskell.org/ghc/ghc/wikis/trac/ghc/browse/..., which is of
  course nonsense, now point to the correct location in the GitLab
  source code viewer:
  https://gitlab.haskell.org/bgamari/gitlab-migration/issues/53
- Some remaining links to the old Trac install have been fixed:
  https://gitlab.haskell.org/bgamari/gitlab-migration/issues/26
- Some incorrectly quoted blocks may or may not be fixed by this as
  well:
  https://gitlab.haskell.org/bgamari/gitlab-migration/issues/46
- During the original import, we experienced HTTP errors on some pages,
  leading to "CONVERSION ERROR" pages being rendered instead. The re-run
  should have fixed these by sheer virtue of GETting those pages again
  now that there is less load on the Trac server.

Since we used the wiki in its pre-migration state (as currently seen on
https://trac.haskell.org/ghc) as the data source, we have to be careful
not to accidentally undo changes made to the GitLab wiki in the
meantime. I tried to mitigate this by manually picking changes, and I
think I got it right, but as with all manual efforts, it is possible
that I made a mistake here or there. The changeset is pretty large, and
most of it is perfectly boring, so I applied quite a bit of automation
to keep it manageable.

So if you find anything broken on the wiki that looked OK so far, or if
you see any of your edits disappear, please holler, and I'll do my best
to fix it. Fortunately, this is a git repo, so nothing is permanently
lost either way.

And of course I apologize for any inconvenience in advance.

Cheers,

Tobias