Introduce global properties to persist uri's for all significant concept sources

In OpenMRS, concept sources are used to represent code systems such as LOINC and SNOMED.

In FHIR, representing concepts includes the use of a uri for each code system, as described in http://www.hl7.org/implement/standards/fhir/datatypes.html#Coding

However, the OpenMRS data model doesn't store uri's for each code system. And so, we currently don't have a way of identifying the uri for each code system.

As a short term solution, I recommend that we add global properties for the significant code systems such as LOINC and SNOMED, and use these to obtain the uri's for each. If users need to introduce their own code systems, then they are responsible for creating global properties for each of them.

The FHIR recomended uri for each code system can be found here:
http://www.hl7.org/implement/standards/fhir/terminologies-systems.html