feat(BA-543): Add enable_model_folders option to webserver to control Models visibility in WebUI

[agatha197]

resolves #3502 (BA-543)
related PR: lablup/backend.ai-webui#3049

Adds a new configuration option enable_model_folders to control the visibility of the Models tab and MODEL usage mode in the data & storage page. The default value is set to true for backward compatibility.

Checklist:

• Milestone metadata specifying the target backport version
• Mention to the original issue
• Installer updates including:
  • Fixtures for db schema changes
  • New mandatory config options
• Update of end-to-end CLI integration tests in ai.backend.test
• API server-client counterparts (e.g., manager API -> client SDK)
• Test case(s) to:
  • Demonstrate the difference of before/after
  • Demonstrate the flow of abstract/conceptual models with a concrete implementation
• Documentation
  • Contents in the docs directory
  • docstrings in public interfaces and type annotations

[agatha197]

• feat(BA-543): Add enable_model_folders option to webserver to control Models visibility in WebUI #3503 👈 (View in Graphite)
• main

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