Replace custom comment hardcoded HTML with include

Also add custom_scripts.html include for custom comment provider JavaScript that loads in the footer.

Close #2549

Merge branch 'master' of https://github.com/mmistakes/minimal-mistakes

Fix staticman v2/v3 comments (#2351)

According to the most recent docs, `branch` and `endpoint` should be inside `site.comments.staticman` config.

Remove Google+

Google+ is being shutdown on April 2, 2019. Social sharing buttong, comment provider, and author link configs have been removed from the theme.

ref: https://support.google.com/plus/answer/9195133

Add support for utterances (#1966)

* Add support for utterances
* Add utterances config documentation
* Update CHANGELOG and history

Close #1909 

Allow custom Staticman endpoints (#1845)

Add conditionals for showing  reCaptcha keys in form

Upgrade Font Awesome to version 5 (#1446)

Upgrade Font Awesome to version 5 (SVG with JavaScript)

Close #1396

Replace `fieldset` elements with `div.form-group`

Move Staticman v2 comment logic above v1 as it will be deprecated soon.

Add support for Staticman v2 endpoint and reCAPTCHA

Ref: #803

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 

Do not print the comments form (#1195)

Add `.page__comments-form` to "non-printing" selectors in print styles

Merge branch 'zmsp-master'

Fix indentation and formatting

Google+ page comments updated.

Merge branch 'release/4.2.2'

Fix bug - include section for Discourse embedded comments (#825)

Rename `#comments` to something more specific
- Close #582

Merge branch 'release/4.0.4'

Conflicts:
	README.md

Merge branch 'feature/theme-gem' into develop