Improve footer link flexibility

Deprecate hardcoded footer links in favor or new `footer.links` array for any link and in any order. All of Font Awesome's icons are available for use.

```
footer:
  links:
    - label: "Twitter"
      icon: "fab fa-fw fa-twitter-square"
      url: "https://twitter.com/username"
    - label: "GitHub"
      icon: "fab fa-fw fa-github"
      url: "https://github.com/username"
    - label: "Instagram"
      icon: "fab fa-fw fa-instagram"
      url: "https://instagram.com/username"
```

Improve author link flexibility

Deprecate social networking links in `_config.yml` in favor or new `author.links` array for any link and in any order. All of Font Awesome's icons are available for use.

```
author:
  links:
    - label: "Your Website"
      icon: "fas fa-fw fa-link"
      url: "https://your-site.com"
    - label: "Twitter"
      icon: "fab fa-fw fa-twitter-square"
      url: "https://twitter.com/username"
    - label: "GitHub"
      icon: "fab fa-fw fa-github"
      url: "https://github.com/username"
    - label: "Instagram"
      icon: "fab fa-fw fa-instagram"
      url: "https://instagram.com/username"
```

Fixes #1581

Add header `actions` array support for multiple call to action links

```
header:
  overlay_image: /assets/images/unsplash-image-1.jpg
  og_image: /assets/images/page-header-og-image.png
  caption: "Photo credit: [**Unsplash**](https://unsplash.com)"
  actions:
    - label: "Learn More"
      url: "https://unsplash.com"
```

Fixes #1461

Add support to gallery helper for defining column layout

e.g. `{% include gallery id="gallery" layout="half" caption="2 column gallery caption" %}

Fixes #1821

Fix sidebar navigation list toggle.

Prettier managled `@include breakpoint` media queries causing the input/checkbox toggle hack styles to go missing.

Fixes #1819

Update CHANGELOG and history

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

Place `:hover` declarations after `:visited` to fix hover animation

Fixes #1820

Update CHANGELOG and history

Release 4.12.2 💎

Update CHANGELOG and history

Add Italian translation for search (#1793)

Adjust toc indents

Update CHANGELOG and history

Update jekyll-toc to v1.0.5

Allow heading levels 1-6 in table of contents

Adjust minimum and maximum levels to allow all headings and fix CSS to style indentation and nested lists properly in jekyll-toc and Kramdown generated table of contents.

Fixes #1782

Reduce padding and adjust toc `font-size`

Fit more text to handle indentation better.

Ref: #1782

Update CHANGELOG and history

Fix: use relative links for menu items when possible (#1783) (#1784)

Update CHANGELOG and history