Import `widgetui` files

[with-heart]

I've been churning a lot on how to handle widgetui files so I think it'd be good to take a step back and review what's needed.

[!NOTE] This issue covers the default game files only. User file uploads will be covered in a separate issue.

This should be a script run locally (because we can only access game installation files from local):

• [x] import *[pP].json files
  • validate using Panel model from schema
  • if validation succeeds, replace existing files in widgetui
  • if validation fails, report field errors so Panel model can be updated
• [x] import materials.json file
  • validate using Materials model from schema
  • if validation succeeds, replace existing file in widgetui
  • if validation fails, report field errors so Materials model can be updated
• [x] import textures directory
  • replace existing textures directory in site/public
  • remove all non-.png files
  • remove large unrelated directories (campaign, menu/usericons, etc.)
• [x] remove textures directory in widgetui
  • it doesn't make sense for this to live here because it's only used by site

[with-heart]

Closed by #18