Added bare bones functionality sending an email

refs https://github.com/TryGhost/Toolbox/issues/292

- Super happy path scenario which handles sending information with some basic incompatibility information
- It's meant to be built up upon! More to follow :)

ghost/api-version-compatibility-service/lib/api-version-compatibility-service.js

@@ -0,0 +1,16 @@
+class APIVersionCompatibilityService {
+    constructor({sendEmail}) {
+        this.sendEmail = sendEmail;
+    }
+
+    handleMismatch({acceptVersion, contentVersion, userAgent}) {
+        const emailTemplate = `
+            ${userAgent} integration expected Ghost version: ${acceptVersion}
+            Current Ghost version: ${contentVersion}
+        `;
+
+        this.sendEmail(emailTemplate);
+    }
+}
+
+module.exports = APIVersionCompatibilityService;

ghost/api-version-compatibility-service/test/api-version-compatibility-service.test.js

@@ -0,0 +1,22 @@
+const assert = require('assert');
+const sinon = require('sinon');
+const APIVersionCompatibilityService = require('../index');
+
+describe('APIVersionCompatibilityService', function () {
+    it('Sends an email to the instance owner when fresh accept-version header mismatch detected', async function () {
+        const sendEmail = sinon.spy();
+        const compatibilityService = new APIVersionCompatibilityService({
+            sendEmail
+        });
+
+        await compatibilityService.handleMismatch({
+            acceptVersion: 'v4.5',
+            contentVersion: 'v5.1',
+            userAgent: 'Elaborate Fox'
+        });
+
+        assert.equal(sendEmail.called, true);
+        assert.match(sendEmail.args[0][0], /Elaborate Fox integration expected Ghost version: v4.5/);
+        assert.match(sendEmail.args[0][0], /Current Ghost version: v5.1/);
+    });
+});