Refactor frontend to remove second iframe

[juliusknorr]

I did a small PoC in https://github.com/nextcloud/richdocuments/compare/chore/refactor-iframe to remove the need for the separate document frame that is being loaded and instead load the Collabora page directly in the first iframe. Instead of loading the intermediate frame it just queries and endpoint to obtain the WOPI token and can then directly pass it to Collabora.

This has several benefits:

• Reduces code complexity as we no longer need to proxy messages through two frames
• No need to keep document state in two different script contexts
• Speeds up loading time by a full page load with Nextcloud javascript

In general this works nicely, but needs a bit of work in order to make sure all previous functionality is still available:

• Basic PoC for viewer usage
  • File creation from templates
• Public share link (folder)
  • File creation from templates
• Public share link (single file)
• Direct editing logged in
  • File creation from templates
• Direct editing share link
• Federated editing
• Make sure all post messages are handled as before