Add issue tracker item to SSSOM datamodel

src/sssom_schema/schema/sssom_schema.yaml

@@ -479,6 +479,18 @@ slots:
     examples:
       - value: https://www.wikidata.org/wiki/Q865360
         description: (the Wikidata identifier for the Jaccard index measure).
+  issue_tracker_item:
+    description: The issue tracker item discussing this mapping.
+    range: EntityReference
+    examples:
+      - value: SSSOM_GITHUB_ISSUE:166
+        description: (A URL resolving to an issue discussing a new SSSOM element request)
+  issue_tracker:
+    description: A URL location of the issue tracker for this entity.
+    range: uri
+    examples:
+      - value: https://github.com/mapping-commons/mh_mapping_initiative/issues
+        description: (A URL resolving to the issue tracker of the Mouse-Human mapping initiative)
   see_also:
     description: A URL specific for the mapping instance. E.g. for kboom we have a
       per-mapping image that shows surrounding axioms that drive probability. Could
@@ -527,6 +539,7 @@ classes:
     - subject_preprocessing
     - object_preprocessing
     - see_also
+    - issue_tracker
     - other
     - comment
   mapping:
@@ -573,6 +586,7 @@ classes:
     - semantic_similarity_score
     - semantic_similarity_measure
     - see_also
+    - issue_tracker_item
     - other
     - comment
     class_uri: owl:Axiom
@@ -587,6 +601,7 @@ classes:
       - mapping_set_references
       - documentation
       - homepage
+      - issue_tracker
   mapping set reference:
     description: A reference to a mapping set. It allows to augment mapping 
       set metadata from the perspective of the registry, for example, providing