Change autolinked URLs to output nice quotes

ref #4257

- This is a follow on from #4257 where I updated the codestyle.
- We were outputting escaped single quotes when double quotes would be much nicer.
- Changes the HTML output from our markdown conversion (non-breaking change)
- Updated tests to match the new output
Merge pull request #4280 from ErisDS/gfm

Change autolinked URLs to output nice quotes