Publish as gem theme

## Prerequisites
- [X] 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](https://jekyllrb.com/docs/themes/#creating-a-gem-based-theme) so we can just reference it in github.com/elementary/blog and use it like any other theme.