Publish as gem theme

[btkostner]

Prerequisites

• I have searched open and closed issues for duplicates.

Feature

Is your feature request related to a problem? Please describe.
The github.com/elementary/blog repository duplicates a lot of this work making it hard to sync changes.

Describe the solution you'd like
Publish the github.com/elementary/blog-template as an actual gem jekyll theme so we can just reference it in github.com/elementary/blog and use it like any other theme.

[cassidyjames]

This sounds like a great solution! 👍