Fix schema.org dates to ISO-8601 (#2339)

* Fix schema.org dates to ISO-8601

* Revert date_to_xmlschema in <time> tag

Change remaining schema.org markup to `https` (#1978)

Replace modified with last_modified_at (#930)

Fixes #929

Merge branch 'release/4.2.0' into develop

Merge branch 'release/4.2.0'

Add video header support to `archive` and `splash` layouts

Merge branch 'release/4.0.4'

Conflicts:
	README.md

Merge branch 'feature/theme-gem' into develop

Replace base_path with `absolute_url` filter where possible

Merge branch 'release/3.4.4'

Merge branch 'feature/fix-page-hero-liquid-warning' into develop

Merge branch 'hotfix' of https://github.com/vwchu/minimal-mistakes into feature/fix-page-hero-liquid-warning

Fix Liquid syntax error for offending parentheses

Liquid Warning: Liquid syntax error (line 3): Expected dotdot but found id in "(page.header.overlay_color or page.header.overlay_image) or page.header.image" in /_layouts/single.html
Liquid Warning: Liquid syntax error (line 1): Expected dotdot but found id in "(page.header.overlay_color or page.header.overlay_image) or page.header.image" in /_layouts/archive.html
Liquid Warning: Liquid syntax error (line 3): Expected dotdot but found id in "(page.header.overlay_color or page.header.overlay_image) or page.header.image" in /_layouts/splash.html

Merge branch 'release/3.0.0'

Merge branch 'feature/spit-and-polish' into develop

Add header overlay sample posts

Rough out splash page _layout with styles and test content