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 

Add enhancements to Unreleased

Convert some links from HTTP to HTTPS (#1234)

Update CHANGELOG and history

Rename `gems` key to `plugins` (#1239)

`gems` key in `_config.yml` was deprecated in Jekyll 3.5 and changed to `plugins`.

ref: https://github.com/jekyll/jekyll/pull/5130

Update CHANGELOG and history

Add link to #1213

Bump :gem: to 4.5.2

Truncate archive item titles' that overflow with an ellipsis

Fixes #1213

Show 4 latest posts when `site.related_posts` is `nil`

`site.related_posts` should return an array of latest posts when no related ones are found due to `lsi` being disabled (GitHub Page's doesn't support it).

This works around that bug by showing the 4 latest posts to populate the "You May Also Enjoy" module when enabled with `related: true`.

Fixes #554
Related: #978 #1022

Update CHANGELOG and history

Fix Liquid syntax error in `feature_row` include

Fix "Expected id but found open_square in `"{{page.[include.id] }}"`" in `feature_row` include.

Liquid syntax error: "Expected end_of_string but found pipe in `"name in __names | sort"`" in `group-by-array` include

`sort` not allowed in `for` statements, remove it.

Fix Liquid syntax error: Expected id but found open_square in `"{{page.[include.id] }}"`

Update CHANGELOG and history

Fix Liquid errors when in `error_mode: strict`

Liquid does not allow parenthesis, remove them to fix Liquid Exception: Liquid syntax error "Expected dotdot but found pipe" in `tag-list.html` and `category-list.html`

Close #1223

Update CHANGELOG and history

Update CHANGELOG and history

Improve translation for minute_read. (#1211)

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