scss lint - aka "automate most of my job"

Here is a first go at automating my job (reviewing indentation levels and whitespace in our growing Sass codebase).

For now, you may run `grunt validate:sass` locally to check if your files validate against our coding conventions.

I made the necessary changes so that most of our .scss files now pass the linter (hence the big pull request), but a few linting errors remain.

#### Remaining linting errors

```
common/app/assets/stylesheets/module/_identity.scss:5 [W] Selector `#header.identity-header` can be simplified to `#header`, since IDs should be uniquely identifying
common/app/assets/stylesheets/module/external/_from-content-api.scss:232 [W] Merge root rule `.from-content-api` with identical rule on line 12
common/app/assets/stylesheets/module/external/_from-content-api.scss:258 [W] Merge root rule `.from-content-api` with identical rule on line 12
common/app/assets/stylesheets/module/external/_from-content-api.scss:292 [W] Merge root rule `.from-content-api` with identical rule on line 12
common/app/assets/stylesheets/module/football/_stats.scss:184 [W] Merge root rule `.match-stats--darken-away` with identical rule on line 157
common/app/assets/stylesheets/module/football/_stats.scss:190 [W] Merge root rule `.match-stats--darken-home` with identical rule on line 147
```

If you work in identity or football, it would be great if you had a look at these. I'll have a look into the errors in `_from-content-api.scss`.

#### More to come soon:
- documentation
- pre-commit hook

![ ](http://media2.giphy.com/media/dw5a7imGVTKxO/200.gif)