Support SEPA

service/config/sample.yml

...rg/whispersystems/textsecuregcm/WhisperServerService.java

...s/textsecuregcm/configuration/BraintreeConfiguration.java

...securegcm/configuration/OneTimeDonationConfiguration.java

...tems/textsecuregcm/configuration/StripeConfiguration.java

...ems/textsecuregcm/controllers/SubscriptionController.java

...systems/textsecuregcm/subscriptions/BraintreeManager.java

...persystems/textsecuregcm/subscriptions/PaymentMethod.java

...persystems/textsecuregcm/subscriptions/StripeManager.java

...securegcm/subscriptions/SubscriptionProcessorManager.java

...textsecuregcm/controllers/SubscriptionControllerTest.java

...ems/textsecuregcm/subscriptions/BraintreeManagerTest.java