[landing][web] Migrate from assets-webpack-plugin to webpack-manifest-plugin

Summary:
This addresses ENG-3328. assets-webpack-plugin has this problem and webpack-manifest-plugin is officially recommended by Webpack.

Old JSON file looked like this:

{"browser":{"css":"prod.41af96d9c587.build.css","js":"prod.8860cda0687b.build.js"},"":{"wasm":"olm.983876380b11.wasm"}}

New JSON file looks like this:

{
  "browser.css": "prod.41af96d9c587.build.css",
  "browser.js": "prod.3bf3c62a7a70.build.js",
  "olm.wasm": "olm.983876380b11.wasm"
}

Test Plan: I ran keyserver / web / landing in prod mode and confirmed that JS, CSS, and olm.wasm all successfully loaded

Reviewers: atul, kamil, michal

Reviewed By: michal

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7072