Decouple PRService from digger config reading

[ZIJ]

Currently PRService is created as part of reading digger config
Which doesn't allow to report an error to the user if smth goes wrong in reading digger config
That is bad because at this point there is no other way to report an error (the job hasn't started yet)
So we want to refactor it to be able to report errors as comments as early as possible

digger/backend/controllers/github.go

Line 414 in 5cb8a94

ghService, config, projectsGraph, branch, err := getDiggerConfig(gh, installationId, repoFullName, repoOwner, repoName, cloneURL, prNumber)