Backfill note subscriptions

[AntonKhorev]

Part two of #5283.

Creates a subscription for every note commented by a non-anonymous user. Does this by:

• iterating through batches of note comments
• getting distinct non-null (user, note) pairs
• inserting these pairs using upsert_all, this skips already existing pairs

[tomhughes]

This looks good now, thanks.