scss lint

I have linted the entire framework now and updated the linting rules. This means minor changes to many of the files and one slightly larger refactor to `_tables.scss`. I could not remove all warnings and errors without a larger refactor, which will probably happen quite soon.

The linting rules are in line with Kickoff's style and should be seen as a guide for your own use.

See below for the existing warnings and errors:

```sh
grunt scsslint
Running "scsslint:allFiles" (scsslint) task
Running scss-lint on allFiles
scss/_typography.scss:24 [W] VendorPrefixes: Avoid vendor prefixes.
scss/mixins/_linear-gradient.scss:23 [W] VendorPrefixes: Avoid vendor prefixes.
scss/mixins/_linear-gradient.scss:24 [W] VendorPrefixes: Avoid vendor prefixes.
scss/mixins/_linear-gradient.scss:25 [W] VendorPrefixes: Avoid vendor prefixes.
scss/mixins/_linear-gradient.scss:26 [W] VendorPrefixes: Avoid vendor prefixes.
scss/mixins/_radial-gradient.scss:10 [W] VendorPrefixes: Avoid vendor prefixes.
scss/mixins/_radial-gradient.scss:11 [W] VendorPrefixes: Avoid vendor prefixes.
scss/mixins/_radial-gradient.scss:12 [W] VendorPrefixes: Avoid vendor prefixes.
scss/mixins/_radial-gradient.scss:13 [W] VendorPrefixes: Avoid vendor prefixes.
scss/mixins/_utility.scss:86 [W] VendorPrefixes: Avoid vendor prefixes.
scss/mixins/_utility.scss:97 [W] VendorPrefixes: Avoid vendor prefixes.
scss/partials/components/_forms-custom-controls.scss:137 [W] MergeableSelector: Merge rule `.form-input--selectWrapper select` with rule on line 130
scss/partials/components/_forms-custom-controls.scss:177 [W] UrlQuotes: URLs should be enclosed in quotes
scss/partials/components/_forms-custom-controls.scss:177 [W] DuplicateProperty: Property `background` already defined on line 164
scss/partials/components/_forms.scss:64 [W] MergeableSelector: Merge rule `.form-legend + .form-controlGroup` with rule on line 35
scss/partials/components/_tables.scss:42 [E] SelectorDepth: Selector should have depth of applicability no greater than 3, but was 4
scss/partials/components/_tables.scss:42 [E] NestingDepth: Nesting should be no greater than 3, but was 4
scss/partials/components/_tables.scss:149 [E] SelectorDepth: Selector should have depth of applicability no greater than 3, but was 4
scss/partials/components/_tables.scss:193 [E] SelectorDepth: Selector should have depth of applicability no greater than 3, but was 4
scss/partials/components/_tables.scss:193 [E] NestingDepth: Nesting should be no greater than 3, but was 4
scss/partials/components/_tables.scss:208 [E] SelectorDepth: Selector should have depth of applicability no greater than 3, but was 4
scss/partials/components/_tables.scss:208 [E] NestingDepth: Nesting should be no greater than 3, but was 4
scss/partials/components/_tables.scss:213 [E] SelectorDepth: Selector should have depth of applicability no greater than 3, but was 4
scss/partials/components/_tables.scss:213 [E] NestingDepth: Nesting should be no greater than 3, but was 4
scss/partials/components/_tables.scss:222 [E] SelectorDepth: Selector should have depth of applicability no greater than 3, but was 4
scss/partials/components/_tables.scss:224 [E] SelectorDepth: Selector should have depth of applicability no greater than 3, but was 5
scss/partials/components/_tables.scss:224 [E] NestingDepth: Nesting should be no greater than 3, but was 4
scss/styleguide.scss:187 [W] QualifyingElement: Avoid qualifying class selectors with an element.
scss/styleguide.scss:188 [W] QualifyingElement: Avoid qualifying class selectors with an element.
```