Upgrades to Acorn Django libraries to support Django/Wagtail 4.x and beyond:

• Upgrade icons to utilize SVG rather than icon-font values (icon font was removed in Wagtail 2.15)
• Provide support for Wagtail < 4.0.x telepath choosers (wagtail-content#4)
• Upgrade Wagtail Content Vectors application to utilize Wagtail/Django 4.x patterns and provide feature parity with Wagtail Images (wagtail-content#5)
• Refactor dependencies in lib to utilize packages rather than submodule embeds

Associated milestones:

• Rockstar CMS: Headless, Widgets for Foundation, Recommender Engines