[keyserver] Serve .well-known from landing endpoint

Summary:
In mobile app we're using `comm.app` as deep links. To do that, we need to serve `.well-known` on this url (which is associated with our landing page).

https://linear.app/comm/issue/ENG-4185/cant-open-web-app-if-not-already-logged-in-dev#comment-6dba4247

Test Plan: Check if `/landing/.well-known/assetlinks.json` returns a proper file.

Reviewers: kamil, inka, bartek

Reviewed By: bartek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D8357

keyserver/src/keyserver.js

@@ -134,6 +134,17 @@ import {
     if (landingBaseRoutePath) {
       const landingRouter = express.Router();
       landingRouter.get('/invite/:secret', inviteResponder);
+      landingRouter.use(
+        '/.well-known',
+        express.static(
+          '.well-known',
+          // Necessary for apple-app-site-association file
+          {
+            setHeaders: res =>
+              res.setHeader('Content-Type', 'application/json'),
+          },
+        ),
+      );
       landingRouter.use('/images', express.static('images'));
       landingRouter.use('/fonts', express.static('fonts'));
       landingRouter.use(