Add support for remote models (OpenAI, ...)

[cztomsik]

• Add OPEN_AI_KEY, etc. in Settings
• Update <ModelSelect, to check if this is filled and to include OpenAI models
• If any OpenAI model is selected make it visually distinctive, that you are using remote model
• Hide "partial" completion in <EditMessage if remote model is selected
• call remote endpoints
  • these are different to what we are doing right now, so maybe we can first add /api/chat/completions endpoint which will just wrap what we do in client-side and then if we are using remote endpoint, we can just proxy