Fix self url for PreprintsInstitutionsRelationshipSerializer

In addition, removed the superfluous 'html' link that is not
part of JSON:API specs.

api/preprints/serializers.py

@@ -625,14 +625,10 @@ class PreprintsInstitutionsRelationshipSerializer(BaseAPISerializer):
 
     links = LinksField({
         'self': 'get_self_url',
-        'html': 'get_related_url',
     })
 
     def get_self_url(self, obj):
-        return obj['self'].absolute_api_v2_url
-
-    def get_related_url(self, obj):
-        return f"{obj['self'].absolute_api_v2_url}institutions/"
+        return obj['self'].institutions_relationship_url
 
     class Meta:
         type_ = 'institutions'

osf/models/preprint.py

@@ -433,6 +433,10 @@ def display_absolute_url(self):
     def linked_nodes_self_url(self):
         return self.absolute_api_v2_url + 'relationships/node/'
 
+    @property
+    def institutions_relationship_url(self):
+        return self.absolute_api_v2_url + 'relationships/institutions/'
+
     @property
     def admin_contributor_or_group_member_ids(self):
         # Overrides ContributorMixin