Change url and image into synchronous helpers

Closes #3961
Add reentrant conversion to Showdown footnotes.

closes #4668

- inline markup in footnotes now works properly
- note that multi-paragraph footnotes are still broken
Merge pull request #4722 from mbrock/master

Add reentrant conversion to Showdown footnotes.
Merge pull request #4726 from jaswilli/issue-3961

Change {{url}} and {{image}} into synchronous helpers
Merge branch 'master' of https://github.com/TryGhost/Ghost

Conflicts:
	core/server/helpers/index.js