Refactor changelog logic to directly use JSON (#296)

Closes #264 

* Basic refactor

* Refactor into module

* Further refactor

* Use json for group pr body

* Add commit links

* improve body

* Refactor names

* Add signature

* Refactor config

* Simplify non-group pr body template

* Fix Pull/Merge terminology

* Fix test

* Fix pr test

* Fix prettier