Use nicer APIs for toggling a 3PID's shared status

[jryans]

At the moment, when toggling a 3PID's shared status, Riot will:

1. Remove the 3PID from the HS with POST /_matrix/client/r0/account/3pid/delete
2. Re-add the 3PID via the HS with POST /_matrix/client/r0/account/3pid/[email|msisdn]/requestToken and setting the bind flag to the new status
3. Usual 3PID validation and adding flow

This is not great in general, as it's not atomic, so if you never act on the validation token, then you have actually entirely removed the 3PID from your account.

Instead we should change to the new APIs tracked in matrix-org/synapse#5827 and matrix-org/synapse#5862.

To change a 3PID from unshared to shared:

1. Call POST /_matrix/client/r0/account/3pid/[email|msisdn]/requestToken and setting the bind flag to the new status
2. Usual 3PID validation and adding flow

To change a 3PID from shared to unshared:

1. Call POST /_matrix/client/r0/account/3pid/unbind

[jryans]

This concern here is resolved by fixing #2713.