This entity is a syncable template collection, where Courier will maintain a local template collection and mirror template values between.

Added a global template collection manager service.
Added config schema for courier_email template.
Added importTemplate and exportTemplate methods to ChannelInterface.
Renamed pre-existing 'Template collection' to 'Local template collection' to emphasise duality with global template collections.

Added a courier_test_message submodule containing a dummy message entity and associated IdentityChannel plugin.
Adde tests for global template collection entity.
Adde tests for global template collection manager and its operations.