Firebase Stub refactoring, added eslint-plugin-jest dev dependency

__mocks__/gatsby-plugin-firebase/constants/database.js

@@ -0,0 +1,19 @@
+const demoStateResume1Id = 'demo_1';
+const demoStateResume2Id = 'demo_2';
+const initialStateResumeId = 'initst';
+
+class Database {
+  static get demoStateResume1Id() {
+    return demoStateResume1Id;
+  }
+
+  static get demoStateResume2Id() {
+    return demoStateResume2Id;
+  }
+
+  static get initialStateResumeId() {
+    return initialStateResumeId;
+  }
+}
+
+export default Database;