Cache relationships in API (#6482)

* Cache relationships in API

* Fetch relationships for search results in UI

* Only save one account's maps in each cache item

app/models/concerns/relationship_cacheable.rb

@@ -0,0 +1,16 @@
+# frozen_string_literal: true
+
+module RelationshipCacheable
+  extend ActiveSupport::Concern
+
+  included do
+    after_commit :remove_relationship_cache
+  end
+
+  private
+
+  def remove_relationship_cache
+    Rails.cache.delete("relationship:#{account_id}:#{target_account_id}")
+    Rails.cache.delete("relationship:#{target_account_id}:#{account_id}")
+  end
+end