Use DS.Model#eachAttribute in editor-base-controller

Closes #4279
Merge pull request #4281 from novaugust/iss4279

Use DS.Model#eachAttribute in editor-base-controller
Refactor helpers & tests into individual files

no issue

- Split theme helpers into individual files for each
- Do the same for tests
- Have utils to share some things between them
- Move assetHash onto config
Merge pull request #4283 from ErisDS/split-helpers

Refactor helpers & tests into individual files
Merge branch 'master' of https://github.com/TryGhost/Ghost