refs TryGhost/Team#2840
- moves the `entities.decode()` step to the `LinkReplacer` class so that
it's applied to all links, not just the ones that are replaced in the
email service
- adds a test case to `LinkReplacer` to ensure that the
`entities.decode()` step is applied to all links correctly, decoding any
URLs with HTML entities in them
---------
Co-authored-by: Chris Raible <chris@ghost.org>