Clean branches even without a Pull-Request

[blacelle]

see eu.solven.cleanthat.code_provider.github.event.GithubWebhookHandler.computeRelevantBaseBranches(IExternalWebhookRelevancyResult, GitRepoBranchSha1, GithubRepositoryFacade, Optional<GitPrHeadRef>)

For now, we clean branches only if:

1. it is not protected (as per Cleanthat configuration)
2. it is not protected (as per Github rules)
3. is head of at least one PullRequest

This behavior may be unexpected by some customer, which would expect any branch to be cleaned. This may be activated for paying plans (else it may be too costly).

This behavior is also a reason why pull_request events can trigger a clean: as the previous push event 9with no pull_request) did not triggered a cleaning, the pull_request (re)open event should trigger it.