Enable missing models dialog by default

[christian-byrne]

This PR sets the default value of "Comfy.Workflow.ShowMissingModelsWarning" to true. This setting enables the below dialog when loading workflows that have a models field (assuming the user doesn't have the model):

Examples of workflows with model fields can be found in the templates folder of the frontend repo (added in #1076). The format is like this:

    "models": [
      {
        "name": "flux1-schnell-fp8.safetensors",
        "url": "https://huggingface.co/Comfy-Org/flux1-schnell/resolve/main/flux1-schnell-fp8.safetensors?download=true",
        "directory": "checkpoints"
      }
    ],

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