(core) Remove expired attachments every hour and on shutdown

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

app/server/lib/Triggers.ts

@@ -116,7 +116,7 @@ export class DocTriggers {
   public shutdown() {
     this._shuttingDown = true;
     if (!this._sending) {
-      this._redisClient?.quitAsync();
+      this._redisClientField?.quitAsync();
     }
   }