Resolves #739.

- Fixes an issue where Members may not have been able to upload image
files to models because of missing permissions for the
`/class/{class_id}/models` endpoint.
- Adds new `vision_support_override` for overriding the default Vision
support property of the OpenAI model.
- Adds a "No vision capabilities" banner at the top of the Upload files
popover for threads in Azure-powered groups.
- Adds a "Vision capabilities are currently unavailable" explainer Modal
discoverable by clicking on the "No vision capabilities" banner.
- Adds a "Vision capabilities are currently unavailable" banner at the
top of the model selector when selecting a Vision-capable model in
Azure-powered groups.