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.
