* src/engine/api/geary-credentials.vala (Credentials): Refer to auth
tokens, not passwords, so we can use the same object for OAuth2
auth. Update call sites.
* src/engine/api/geary-credentials-mediator.vala (CredentialsMediator):
Refer to auth tokens, not passwords, so we can use the same object for
OAuth2 auth. Remove saving and clearing methods from the interface
since that's only supported for password storage, not SSO like
GOA. Update implementations and call sites.
* src/engine/api/geary-account-information.vala (AccountInformation):
Replace many byzantine auth management methods with just a few for
handling loading and prompting. Remove ServiceFlag enum now it is no
longer used. Clean up call sites.
* src/client/accounts/account-manager.vala (AccountInformation): Manage
initial saving and final deletion of libsecret based accounts here,
rather than all over the code base.
