Automatically close stale batch changes

Created by: malomarrec

Context

Currently, it is possible to close Batch Changes manually when they are considered complete.

Problem

When there are many batch changes over long period of time, it's difficult to know which batch change can be closed, and which are still active and need to stay open. In doubt, batch changes stay open for long, go stale and crowd the UI making it hard to focus on batch changes that need attention.

Teams with heavy batch change usage see this problem most often.

Proposal

(1) Research how a user, from the dashboard, can quickly get a sense of how far along batch changes are, and how active they are. (2) We could display a "stale" batch on a batch change, or a less arbitrary measure of activity. This could be:

• stale means no activity on any PR in the batch change for n days
• we could display when the batch change was active last (defined , for example, by any activity on a changeset in the batch change).
• We probably need to rethink how a user can view "the life of a batch change", in a timeline manner with events such as:
  • opened
  • bulk actions being ran (eg. bulk comments: @ bob requested action)
  • rate of merge at various points in time (eg. 50% closed)
  • a measure of how active the underlying PRs are at various points in time

Impacted customers

This was initially surfaced by Feature requested by https://github.com/sourcegraph/accounts/issues/580, one of our customers with many batch changes.