Let people rename attachments in documents with access rules (#2514)

I didn't know this, but the Grist web UI allows renaming attachments.
This feature broke when combined with access rules. Renaming updates
fileName on _grist_Attachments, a table now locked down against all user
actions once a document has rules to apply. But fileName-only updates
are not actually a concern, so this commit lets them through. For
simplicity, only very simple bundles that match what the web client does
are accepted.

Also includes a tiny fix to rebuild rules when the share table changes,
so removing the last share takes effect without a document reload. Both
small issues hit the same user, which is why they are arriving together
- lmk if I should split.