Makes extension usable with configured alternative file domain

Summary:

• adds library map
• updates readme to
  • use specific draw.io version
  • install extension in another dir than phorge (because that is the cleaner approach)
• reworks the js/css
  • use celerity to load the files from the alternative file domain
  • use javelin behavior to load the diagram data (loadJsExtension route is not needed anymore)
  • adds the toolbar button js/css with javascript from the phorge parent window
• with the last point there is no reason to reverse engineer the contents of the index.html of the draw.io app. We just pass-through the contents from the original index.html and add the toolbar assets from the parent window.
• dumps base64 image data directly while remarkup rendering and adds js/css files for remarkup object rule

Fixes T15782

Test Plan: Install it like described in the readme and test it.

Reviewers: mturdus

Reviewed By: mturdus

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Tags: Diagrams (Extension)

Maniphest Tasks: T15782

Differential Revision: https://we.phorge.it/D25577