Rewrite superfluous issue links
Many issues in the wiki were unnecessarily links:
```
... patch posted in [\#1346](https://gitlab.haskell.org/ghc/ghc/issues/1346).
```
This rewrites these as `#1346` using
```bash
sed -i -r -e 's/\[\\#([0-9]+)\]\([^)]+\/\1\)/#\1/g' $(find -iname '*.md')
```
And manually eyeballing the result.
Updated GHCi support for Tier 2. Removed mention of "registerised" build as we have phased out it's support. Removed mention of foreign import wrapper as it's supported mostly everywhere (native code or libffi).
