Fix and clarify default models

[julien-nc]

• Cleanup admin and user settings
  • Consider all credential/urls as sensitive
  • Always save default models after getting them when credentials/URL have changed
• Fix default model enum values in TaskProcessing providers
• Harmonize default model selection between text and image generation
• Add "model" optional input shape in all text and image generation task types
• Make sure default models are saved if they are not up-to-date when browsing the admin settings (cc @szaimen)

closes #129