Notifications for threads

[clokep]

MSC3771 and MSC3773 make changes to the receipt and notifications architecture for handling threads.

• Develop proof of concept for MSC3771 and MSC3773 together
• MSC3773: Notifications for threads
  • Update the notifications schema to handle a thread field.
  • Update the push rule handling code to attach thread to notifications.
  • Update the notification rotation code to handle unique threads.
  • Update the sync code to return notifications separately for each thread.
• MSC3771: Read receipts for threads
  • Update the /receipt servlet handling for thread.
  • Update the receipt schema to handle thread (and properly handle it when not provided).
  • Update the fetching of receipts to handle unique threads.
  • Update the sync code to return the thread of each receipt.

[clokep]

The current proof of concept for this is on the clokep/thread-read-receipts-poc branch.

[clokep]

I think the last task here is to support stable identifiers, which would be needed for #13954.