Split account_threepid_handler into a msisdn and email versions

[anoadragon453]

We realized we could run into an issue where because Synapse can't support sending SMS yet, you may find yourself wanting to use Synapse to send password reset emails, but then can't support msisdn registration.

We need to split up account_threepid_handler into msisdn and email versions so that people can configure these things separately.

On the backend side, this will involve:

• Updating UPGRADE.rst and other various changelog notes
• Updating the requires_identity_server unstable /versions flag to have an email and msisdn version, so clients know what to do for each
• Updating the account_threepid_handler config option and creating two, and updating all the checks to either be based off the msisdn or the email version

[anoadragon453]

Done in #5940