Setting sections are slightly confusing when empty

[krassowski]

Description

Specific settings sections (Language model, Embedding model, Inline completions model, API Keys) may be empty if:

• the operator disabled all models of given type
• user has no API keys configured and selected model does not require any

This leads to a rather confusing display of empty sections because user may expect to be able to make a choice or see an imput for API keys, but is presented with an empty dropdown or API keys empty section without any explanation.

Reproduce

1. jupyter lab --AiExtension.allowed_providers=''
2. Open chat settings

Expected behavior

1. The API keys section is either hidden or shows "No API keys needed for the selected models" placeholder
2. The model sections are hidden if no relevant model is found, or instead of a dropdown list show a placeholder like "No embedding models are configured by the system administrator" (wording to be improved) to highlight that this is not a user error and user has nothing to do other than contact the operator of the Jupyter instance if they really want such a model.

Context

2.19.0