Basic structure of the project

- Added a model with in-memory database, and routes, controller and
middleware to enable making initial CRUD calls to endpoints;

- Added jest and an initial test.

src/models/topic.ts

@@ -0,0 +1,11 @@
+export interface Topic {
+  id: number;
+  name: string;
+  content: string;
+  createdAt: string;
+  updatedAt: string;
+  version: number;
+  parentTopicId: number;
+}
+
+export let topics: Topic[] = [];