The dashboard should unsubscribe to the underlying data when clearing a table

[sujayakar]

We clear a table by repeatedly sending mutations from the client to delete chunks from the table. We added a cursor for the mutations to avoid issues with retention where we scan over all the documents we just deleted.

However we're still subscribed to the query that lists the documents in the table that hits these retention issues. We should probably unsubscribe from this query while the delete is happening / stop showing the data page in the background.

I think we'd run into a similar issue if someone immediately went back to the data page for that table after clearing their table, and I'm less clear how we could fix that.

[Medard1994]

good