Add automatic cleanup script.
Add automatic cleanup script.

Summary:
Added a script to:
- Find inactive sources (no new entries in last 24 hours) and mark them as such (setting endTime to timestamp of last log).
- Prune inactive sources with no associated log entries.

Test Plan: TODO. Also should add a cronjob and install it in the Dockerfile; not sure what the easiest way to do that is. Alternatively could make this part of the main Flask app, but I'm somewhat wary of that approach.

Reviewers: girts

Reviewed By: girts

Differential Revision: D408

astranis/starlog,astranis/starlog,astranis/starlog,astranis/starlog