One part of SystemNoteService refactoring.

Move specs to SystemNotes::CommitService

Add BaseService.

Preparation for the follow-up MRs
regarding SystemNoteService refactoring.
Specs are using doubles

Also adding specs for BaseService and
changed scope of some methods to protected.

Brought back accidentally removed comment
to document return value.

Use expect_next_instance_of instead of 
expect_any_instance_of

Apply suggestion to 
spec/services/system_note_service_spec.rb

Apply suggestion to 
spec/services/system_note_service_spec.rb