insights: various critical bug fixes & improvements

Created by: slimsag

1. Make the backend store insights per-repository. The historical enqueuer does this today, but not the regular enqueuer because we needed to extract out match counts per repository and this is a bit annoying to do. Now implemented. Helps #18966 (closed) and makes it clear how we'd implement querying of per-repo datapoints this in the future.

2. When considering backfilling data for certain timeframe, check if the specific repo we're backfilling data for has any. This should hopefully fix the issue on Sourcegraph.com and k8s.sgdev.org where the graph is totally flat which is very incorrect. Suspicion is that it managed to generate one data point correctly, ran into an error, restarted, and now thinks "we have data for those timeframes" Helps #18961 (closed)

3. Ignore cached repos that are not found, e.g. if someone actually removes a repo and we have it in our cache to iterate over ignore it. Fixes an issue Thorsten noticed https://sourcegraph.slack.com/archives/CHPC7UX16/p1615468766294800