Container editor

[LoveIsGrief]

Resolves #122 and #60. Introduces a simpler container editor that will allow creating new containers, editing or deleting existing ones.

Screen recording

TODO:

• [x] Create new container
• [x] Edit existing container (Preview)
• [x] Delete existing container
• Add checks before creating and updating containers
  • [x] Check for dupes
  • [x] Handle errors
• Clean up code
  • [ ] Use web components or forms
  • [x] Use CSS mask-image instead of convoluted filter + fill
• UI improvements
  • [x] Visual feedback for invalid forms
  • [x] Button size in create/edit form