Use GraphQL API for GitHub interactions

[nikku]

Is your feature request related to a problem? Please describe.

GitHub API v4 is a GraphQL one, cf. GraphQL v4.

Using it reduces bandwidth and safeguards us from accidential API miss-use (core feature in GraphQL).

Describe the solution you'd like

• Current usages of the REST API are replaced with GraphQL calls
• We are as good with regards to rate limits as we are with the current REST API calls

Describe alternatives you've considered

Keep things untouched. The REST API will probably stay around for a long time.