Refactor: mapping creation and mappings tests

[syphax-bouazzouni]

Prerequisite

• https://github.com/ncbo/ontologies_api/pull/101

Changes

• simplify the post (creation of a new mapping) in the mappings_controller.rb, by re-using LinkedData::Mappings.create_mapping and models validations
• update mappings tests to use the new mappings model (added two fields subject_source_id and object_source_id)