Jekyll 4, jekyll-pagination-v2, gem theme

[karrirasinmaki]

How about creating gemspec for this theme to make it gem theme?

Also, Jekyll could be updated to version 4 and jekyll-pagination to jekyll-pagination-v2.

This PR is not intended to be merged as it is, but rather work as a discussion opener.

---

In this PR:

• Upgraded: Jekyll 3.8.4 to 4.0.0
• Upgraded: jekyll-paginate to jekyll-paginate-v2
• Upgraded: compress.html to v3.1.0
• Changed: _config.yml to support new jekyll-paginate-v2
• Changed: hard-coded strings into the separate language file
• Changed: a few changes in code for easier customization from child theme
• Added: gemspec to allow installing theme as a gem