[keyserver][landing] Fix reset preamble for landing SIWE

Summary:
To address `incognito={true}` [not working on Android](https://linear.app/comm/issue/ENG-2577/siwe-webview-incognito-property-not-working-on-android), in D6112 I introduced a "preamble" of sorts to `landing` that would reset `localStorage`.

Sometime since then this has stopped working (see [ENG-9537](https://linear.app/comm/issue/ENG-9537/investigate-issues-with-zerion-wallet)). This diff makes two changes to bring it back:

1. I had to move the preamble to a separate `<script>` tag in the returned HTML. I'm not sure why this was necessary, as my naive prior was that Webpack would execute the first lines of your `entry` first. But testing clearly revealed this to be necessary.
2. I also had to add some lines to clear IndexedDB alongside `localStorage`.

Test Plan:
I created a test environment on my personal server at https://comm.fund and then authorized comm.fund via WalletConnect's "Cloud" interface

I then deployed a dev build to my local device pointed at comm.fund/siwe. I confirmed that this worked by testing 10x times that swiping away `SIWEPanel` after initially connecting a wallet would result in a full reset the next time `SIWEPanel` was opened

I then tested the Android Zerion wallet workflow end-to-end

Reviewers: varun, will

Reviewed By: will

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13789

keyserver/src/responders/landing-handler.js

@@ -198,6 +198,14 @@ async function landingResponder(req: $Request, res: $Response) {
         <base href="${basePath}" />
         ${fontsInclude}
         ${cssInclude}
+        <script>
+          localStorage.clear();
+          indexedDB.databases().then(databases => {
+            databases.forEach(db => {
+              indexedDB.deleteDatabase(db.name);
+            });
+          });
+        </script>
         <link
           rel="apple-touch-icon"
           sizes="180x180"

landing/script.js

@@ -1,7 +1,5 @@
 // @flow
 
-localStorage.clear();
-
 import 'isomorphic-fetch';
 
 import invariant from 'invariant';