Email notification pushers removed on signout

This issue has been migrated from [#3696](https://github.com/matrix-org/synapse/issues/3696).

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There are two ways to set up email notifications for your account:
* Upon registration (via configuration options)
* By setting up an `m.email` pusher (clicking the checkbox in Riot)

Both of these are tied to the access token that was used to create them. For registration, this is the first access token for the user. If the user logs out to invalidate the access token, the email pusher goes away. This decision makes sense for HTTP pushers, given logging out of an app should surely stop sending data to it, however for email it feels a bit awkward. It seems like a valid use case to not want to be logged in to your matrix account anywhere, and instead receive an email when someone needs you - but this cannot be done without leaving a device logged in somewhere.