Move email credentials into CredentialsStore, update schema

It is better to have credentials as a separate OAuth scope, and to
be able to encrypt them with a master password. So far only sockethub
credentials were in a separate module, this PR does the same for email
credentials, which were tagged onto the messages module until now.

This commit also switches to the new CredentialsStore class, and updates
the schema to match the latest API changes in sockethub.