Refactor pull request handling

[AlexanderLanin]

This pulls the PullRequest class one level up, so it can use the GraphQL API as well. New features that would benefit from this are not included in this PR. (The code is already there in different places, it's just not in the PullRequest class)

In order to achieve that, users like InstallationBasedTask must not use rest_api and graphql_api directly. They must use GitHubProvider abstraction level.

Hint: see individual commits for review. Unfurtunately quite a lot of code needed to be touched here.

⚠️ I was not able to test the webapp functionality. Ideally this is just refactoring, and absolutely no behavior change, But the last commit is a good example of stuff that mypy did not catch :(

Resolves #604

• This is draft, as I accidentally included feat: comment on auto-merge problems #603 on the branch. Easiest would be to rebase once feat: comment on auto-merge problems #603 is merged. --