Implement backend changes to enable default credentials with the following rules:

• Default credentials will only be available for Source Control and Maven Settings file credential types, at least on this first iteration.
• There can only be one default credential per credential type.
• Once a credential is set as default, it will be implicitly associated to all applications that don't have a credential of that type explicitly associated.
• Changing the default credential for a certain credential type should trigger discovery for all applications that don't have a credential of that type explicitly associated.
• UI should be explicit about default credentials being used by an application.

Full functional specification is available in the linked story.