Store jobs of pending workflows in memory

[Kobzol]

It would be nice to show which CI jobs are still being computed (are pending) for the currently pending workflow on the queue page. However, given that this is just a nice-to-have feature, I don't want to complicate it by involving the DB. So I would just add a new webhook listener that will listen for job changes, and store their state in memory, in a non-persistent way, probably in the GitHubRepositoryClient struct (so that it is per repo). There should be some LRU cache or something, to avoid it consuming a lot of memory. In reality, we always care only about the jobs of a pending workflow, so we can simply dump all jobs of workflows that are already completed.