Tweaking SCSS to pass scss-lint

Should now pass [scss-lint](https://github.com/causes/scss-lint) (with a minor exception, see below).

Diff is a bit ugly since I converted from 4 spaces to 2 spaces. To view diff without whitespace diffs, use 

The one minor exception to a clean `scss-lint` is any .scss file with YAML front matter (lib/site_template/css/main.scss, for example), will fail the linter because YAML isn't valid in SCSS. I manually removed the YAML front matter and ran the linter to make sure everything was kosher, and then pasted the YAML front matter back in.

```sh
$ scss-lint lib/site_template/**/*.scss
lib/site_template/css/main.scss:1 [E] Invalid CSS after "-": expected number or function, was "--"
$ echo $?
65

$ scss-lint lib/site_template/_sass/*.scss
$ echo $?
0
```

The `scss-lint` rules can be found in `.scss-lint.yml` config file below. Feel free to make edits to the rules, or just reject this PR. **¯\\_(ツ)_/¯**

Fixes #2890