Extract platform code

[bwindels]

• Move ui/web into platform folder, so things now in ui/web/dom can go into platform/web/dom and ui/web/... can go into platform/web/ui
• move all things we inject into the session container into a platform object. This is both platform specific things and non-deterministic global state things like the random function:
  • clock
  • cryptoDriver
  • olm
  • olmWorker
  • onlineStatus
  • storageFactory
  • session info storage
  • request function (xhr.js and fetch.js will be moved out of matrix/net and into platform/web/dom/net/)
  • random function
  • textencoder/decoder as encoding.{toBytes, toUTF8}
  • base64/base58 encoding/decoding?
  • given that Uint8Array is universally supported, it can be used everywhere and not just in platform specific code.
• for web, both the modern and legacy bundle export a Platform class next to the main. Dependencies that should only be in either bundle should be exposed through this platform class (e.g. AES-CTR not being natively supported on legacy). E.g. there will be a platform/web/LegacyPlatform.js and a platform/web/ModernPlatform.js which will export a Platform symbol.
• the main function receives a platform argument, an instance of the exported Platform class, which it will pass to the session container.
• platform also creates the root view given the root view model