Summary:
Call ActiveDoc.removeUnusedAttachments every hour using setInterval, and in ActiveDoc.shutdown (which also clears said interval).
Unrelated: small fix to my webhooks code which was creating a redis client on shutdown just to quit it.
Test Plan:
Tweaked DocApi test to remove expired attachments by force-reloading the doc, so that it removes them during shutdown. Extracted a new testing endpoint /verifyFiles to support this test (previously running that code only happened with `/removeUnused?verifyfiles=1`).
Tested the setInterval part manually.
Reviewers: paulfitz, dsagal
Reviewed By: paulfitz
Subscribers: dsagal
Differential Revision: https://phab.getgrist.com/D3387