Color "skins" (#1242)

* Add YIQ Color Contrast mixin
* Remove text underline from `.btn` links
* Move YIQ variables
* Simplify button classes using YIQ color contrast mixin
  - DRY up button CSS using Sass list and YIQ color contrast mixin.
  - Move `color` and `background-color` to new `btn--primary` class instead of assinging on the default class. Removes the need to override them.
* Add `.btn--primary` to buttons that just had `.btn`
* Apply changes to `/docs`
* Add `.btn--primary` class
* Update CHANGELOG and history
* Add sample form
* Abstract colors away into Sass variables for easier themeing
* Add "dark" skin
* Replace hardcoded color with SCSS variable
* Invert Font Awesome icons' colors in author sidebar and footer
* Add Sass changes to `/docs`
* Use primary button type instead of inverse
* Add missing `!default` on `$muted-text-color`
* Add `contrast` and `sunrise` skin colors
* Add `dirt` skin color
* Add `air` skin color option
* Add `mint` skin color
* Add `btn--primary` class to Submit Comment button
* Set skin to `default`
* Document skin color options
* Add note about skin SCSS import

Close #1208

docs/_pages/splash-page.md

@@ -24,7 +24,7 @@ feature_row:
     excerpt: "This is some sample content that goes here with **Markdown** formatting."
     url: "#test-link"
     btn_label: "Read More"
-    btn_class: "btn--inverse"
+    btn_class: "btn--primary"
   - image_path: /assets/images/unsplash-gallery-image-3-th.jpg
     title: "Placeholder 3"
     excerpt: "This is some sample content that goes here with **Markdown** formatting."
@@ -35,7 +35,7 @@ feature_row2:
     excerpt: 'This is some sample content that goes here with **Markdown** formatting. Left aligned with `type="left"`'
     url: "#test-link"
     btn_label: "Read More"
-    btn_class: "btn--inverse"
+    btn_class: "btn--primary"
 feature_row3:
   - image_path: /assets/images/unsplash-gallery-image-2-th.jpg
     alt: "placeholder image 2"
@@ -43,7 +43,7 @@ feature_row3:
     excerpt: 'This is some sample content that goes here with **Markdown** formatting. Right aligned with `type="right"`'
     url: "#test-link"
     btn_label: "Read More"
-    btn_class: "btn--inverse"
+    btn_class: "btn--primary"
 feature_row4:
   - image_path: /assets/images/unsplash-gallery-image-2-th.jpg
     alt: "placeholder image 2"
@@ -51,7 +51,7 @@ feature_row4:
     excerpt: 'This is some sample content that goes here with **Markdown** formatting. Centered with `type="center"`'
     url: "#test-link"
     btn_label: "Read More"
-    btn_class: "btn--inverse"
+    btn_class: "btn--primary"
 ---
 
 {% include feature_row id="intro" type="center" %}