(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

sandbox/grist/migrations.py

@@ -699,6 +699,12 @@ def migration17(tdset):
       actions.ModifyColumn(tables_map[c.parentId].tableId, c.colId, {'type': 'Attachments'})
       for c in affected_cols
     )
+    # Update the types in the metadata tables
+    doc_actions.append(actions.BulkUpdateRecord(
+      '_grist_Tables_column',
+      [c.id for c in affected_cols],
+      {'type': ['Attachments' for c in affected_cols]}
+    ))
     # Update the values to lists
     for c in affected_cols:
       if c.isFormula:
@@ -710,12 +716,6 @@ def migration17(tdset):
         actions.BulkUpdateRecord(table_id, table.row_ids,
           {c.colId: [conv(val) for val in table.columns[c.colId]]})
       )
-    # Update the types in the metadata tables
-    doc_actions.append(actions.BulkUpdateRecord(
-      '_grist_Tables_column',
-      [c.id for c in affected_cols],
-      {'type': ['Attachments' for c in affected_cols]}
-    ))
 
   return tdset.apply_doc_actions(doc_actions)