Implement external issue tracker base model.

app/models/external_issue.rb

+class ExternalIssue
+  def initialize(issue_identifier, project)
+    @issue_identifier, @project = issue_identifier, project
+  end
+
+  def to_s
+    @issue_identifier.to_s
+  end
+
+  def id
+    @issue_identifier.to_s
+  end
+
+  def iid
+    @issue_identifier.to_s
+  end
+
+  def ==(other)
+    other.is_a?(self.class) && (to_s == other.to_s)
+  end
+
+  def project
+    @project
+  end
+end

app/models/jira_issue.rb

-class JiraIssue
-  def initialize(issue_identifier)
-    @issue_identifier = issue_identifier
-  end
-
-  def to_s
-    @issue_identifier.to_s
-  end
-
-  def id
-    @issue_identifier.to_s
-  end
-
-  def iid
-    @issue_identifier.to_s
-  end
-
-  def ==(other)
-    other.is_a?(self.class) && (to_s == other.to_s)
-  end
+class JiraIssue < ExternalIssue
 end