Shorter user slugs (if possible)

Closes #4211
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
Fix up labels for controls in post settings menu

No issue
Merge pull request #4288 from jaswilli/psm-labels

Fix up labels for controls in post settings menu
Merge pull request #4220 from felixrieseberg/iss4211

Shorter user slugs (if possible)
Show all invited users and infinite scroll active

Closes #4239
Update to use libsass 3.0

No issue

This doesn't give us any advantages at the moment, but is less buggy (in edge cases) and ;gets us be more awesome in the future.

Look at https://github.com/sass/libsass/releases/tag/3.0 to see what's in Libsass 3.0
Merge pull request #4296 from PaulAdamDavis/update-libsass

Update to use libsass 3.0
Merge pull request #4289 from jaswilli/issue-4239

Show all invited users and infinite scroll active
Adds Twitter cards to {{ghost_head}}

Issue #3900

- Adds twitter cards to ghost head helper
- Updates test to check for twitter card data
- Updates privacy.md
- Fixes issue with image urls that are linked by url rather than uploaded