chore(styles): configure sass directories

Not sure if we want to standardize on single quotes vs double quotes.

For giggles, here's my local output from my favorite [scss-lint](https://github.com/causes/scss-lint) tool:

```sh
$ scss-lint app/styles/
app/styles/_layout.scss:3 [W] Color `white` should be written in hexadecimal form as `#ffffff`
app/styles/_modules.scss:1 [W] Prefer single quoted strings
app/styles/_modules.scss:2 [W] Prefer single quoted strings
app/styles/_variables.scss:5 [W] Color `#0BC7B1` should be written as `#0bc7b1`
app/styles/_variables.scss:7 [W] Color `#F2F2F2` should be written as `#f2f2f2`
app/styles/_variables.scss:8 [W] Color `#EEE` should be written as `#eee`
app/styles/_variables.scss:11 [W] Color `#00FF00` should be written as `#00ff00`
app/styles/_variables.scss:11 [W] Color `#00FF00` should be written as `#0F0`
app/styles/_variables.scss:12 [W] Color `#FF0000` should be written as `#F00`
app/styles/_variables.scss:12 [W] Color `#FF0000` should be written as `#ff0000`
app/styles/modules/_global-nav.scss:4 [W] Colon after property should be followed by one space
```