New "browse" UI

Also, separated out browse, ?meta, and ?module request handlers.

Fixes #82

modules/__tests__/browseFile-test.js

@@ -0,0 +1,34 @@
+import request from 'supertest';
+
+import createServer from '../createServer.js';
+
+describe('A request to browse a file', () => {
+  let server;
+  beforeEach(() => {
+    server = createServer();
+  });
+
+  describe('when the file exists', () => {
+    it('returns an HTML page', done => {
+      request(server)
+        .get('/browse/react@16.8.0/umd/react.production.min.js')
+        .end((err, res) => {
+          expect(res.statusCode).toBe(200);
+          expect(res.headers['content-type']).toMatch(/\btext\/html\b/);
+          done();
+        });
+    });
+  });
+
+  describe('when the file does not exist', () => {
+    it('returns a 404 HTML page', done => {
+      request(server)
+        .get('/browse/react@16.8.0/not-here.js')
+        .end((err, res) => {
+          expect(res.statusCode).toBe(404);
+          expect(res.headers['content-type']).toMatch(/\btext\/html\b/);
+          done();
+        });
+    });
+  });
+});