This creates a new convenience class which consolidates all the Github initialization. It supports dependency injection and facilitates refactoring the persistence along the lines of #1205.
Also ref #1848
* Basic version of commit-status badge added
* Support for case with no common ancestor
* Handle unknown branch
* Service tests with error responses
* Handle unexpected 404 responses from github
* Branch is a base
* Tests reordered
* Using not the newest commit in tests
* Test for checked commit identical with the newest commit in branch
* Code refactoring
* Example for Github commit merge status