Extracts logic from GitHubProjectDataSource

[simonbs]

This PR refactors the GitHubProjectDataSource type to split it into the following two types:

Type	Description
GitHubRepositoryDataSource	Loads Git repositories from GitHub using their GraphQL API.
GitHubProjectDataSource	Depends on GitHubRepositoryDataSource to load repositories and map them to our Project model.

The logic essentially remains the some but now resides in two types instead of one. This separation of the logic makes it easier to test the types and extend the types, for example using the decorator pattern as is the case in #279.