Change UIFeature.thirdPartyId flag to control email and phone independantly

**Is your suggestion related to a problem? Please describe.**
If we want to disable 3pid login/discovery etc, we can do so in the synapse config, and then remove the 3pid section in element-web though the UIFeature.thirdPartyId flag.

BUT this also prevents the user from setting an email address to receive email notifications, which then leaves a fairly useless phone section that just throws 400 errors.

**Describe the solution you'd like**
UIFeature.thirdPartyId could be split into two flags UIFeature.thirdPartyIdMsisdn and UIFeature.thirdPartyIdEmail to control the two separately.