Add the method ExternalIssue#project_id

This method returns the project's ID, making ExternalIssue slightly more
compatible with Issue (which also defines the "project_id" method).

app/models/external_issue.rb

@@ -29,6 +29,15 @@ class ExternalIssue
     @project
   end
+  def project_id
+    @project.id
+  end
+
+  # Pattern used to extract `JIRA-123` issue references from text
+  def self.reference_pattern
+    @reference_pattern ||= %r{(?<issue>\b([A-Z][A-Z0-9_]+-)\d+)}
+  end
+
   def to_reference(_from_project = nil)
     id
   end

spec/models/external_issue_spec.rb

 require 'spec_helper'
 describe ExternalIssue, models: true do
-  let(:project) { double('project', to_reference: 'namespace1/project1') }
+  let(:project) { double('project', id: 1, to_reference: 'namespace1/project1') }
   let(:issue)   { described_class.new('EXT-1234', project) }
   describe 'modules' do
@@ -36,4 +36,10 @@ describe ExternalIssue, models: true do
       end
     end
   end
+
+  describe '#project_id' do
+    it 'returns the ID of the project' do
+      expect(issue.project_id).to eq(project.id)
+    end
+  end
 end