Improve headline hierarchy

Add headline specific Sass variables:

```scss
$h-size-1: 1.563em !default; // ~25.008px
$h-size-2: 1.25em !default; // ~20px
$h-size-3: 1.125em !default; // ~18px
$h-size-4: 1.0625em !default; // ~17px
$h-size-5: 1.03125em !default; // ~16.5px
$h-size-6: 1em !default; // ~16px
```

Fixes #2423

Update CHANGELOG and history

Improve accessiblity of default skin by increasing color contrast

Update CHANGELOG and history

Hide hidden posts from listings (#2345)

Update CHANGELOG, history, and documentation

Add Irish (Gaeilge) locale to ui-text.yml (#2422)

Fix rake vulnerability in .gemspec file

Update CHANGELOG and history

fix: Do not redirect user away from site after they submit a comment (#2402)

Update CHANGELOG and history

Disable box-shadow for radio and checkbox (#2398)

Bump Jekyll gem dependency to `v3.7`

Regression for mmistakes#2332 (#2385)

* Regression for mmistakes#2332

There's already a `relative_url` in place, shouldn't stack up another

* Update CHANGELOG and history

Release 4.18.1 💎

Fix compatibility issue with paginate V2 introduced by mmistakes/minimal-mistakes#2378 (#2381)

Release 4.18.0 💎

Update CHANGELOG and history

Allow using theme without pagination (#2378)

Add links to high resolution skin screenshots

Resolves #2363