This makes it so that we rely on the `device_id` to delete pushers on logout,
instead of relying on the `access_token_id`. This ensures we're not removing
pushers on token refresh, and prepares for a world without access token IDs
(also known as the OIDC).
This actually runs the `set_device_id_for_pushers` background update, which
was forgotten in #13831.
Note that for backwards compatibility it still deletes pushers based on the
`access_token` until the background update finishes.
This simplifies the access token verification logic by removing the `rights`
parameter which was only ever used for the unsubscribe link in email
notifications. The latter has been moved under the `/_synapse` namespace,
since it is not a standard API.
This also makes the email verification link more secure, by embedding the
app_id and pushkey in the macaroon and verifying it. This prevents the user
from tampering the query parameters of that unsubscribe link.
Macaroon generation is refactored:
- Centralised all macaroon generation and verification logic to the
`MacaroonGenerator`
- Moved to `synapse.utils`
- Changed the constructor to require only a `Clock`, hostname, and a secret key
(instead of a full `Homeserver`).
- Added tests for all methods.
