Add support for importing images

closes #4608, #4609

- image handler loads in any image files & figures out where they'll get stored
- image importer has a preprocessor which replaces image paths in
  pertinent spots of post, tag and user models
- image importer stores images, keeping the path where it makes sense
- basic test for the preprocessor
Merge pull request #4690 from ErisDS/image-importer

Add support for importing images
Merge branch 'master' of https://github.com/TryGhost/Ghost