Add admin UI for contexts and label extractor definitions

[Tooyosi]

This PR adds an admin UI for managing contexts and DB-backed label extractor definitions in Kade.

What Changed

Context admin

• adds /admin/contexts list and detail pages
• adds create, edit, and delete flows for contexts
• validates module_name / extractor_name before creating or updating a context
• shows definition state on the contexts index as:
  • Enabled
  • Disabled
  • None

Label extractor definition admin

• adds /admin/label_extractor_definitions list page
• adds create, edit, and delete flows for label extractor definitions
• after editing a definition:
  • returns to the context page if edit started there
  • otherwise returns to the global definitions list

Admin form UX

• adds delete confirmation dialogs
• preserves entered JSON on validation failures
• adds lightweight JSON assistance in admin forms:
  • inline valid/invalid status
  • formatting on blur/submit
  • submit blocking for invalid JSON

[Tooyosi]

I made a video to make the review easier. Please check it out @zwolf @lcjohnso