FEATURE: configure a default LLM model for all features

[keegangeorge]

🔍 Overview

This update adds a setting to the configure a default LLM model to be used for all features (unless overridden) and removes the custom: prefix when defining LLM models

➕ More information

For all features, the feature's assigned AI Persona's default_llm_id will be used, however, if it doesn't exist, the SiteSetting.ai_default_llm_model will be used instead.

This update also adds a few migrations which migrates the old model settings for Helper and Summarization into their respective personas. That is:

• SiteSetting.ai_helper_model's existing value on sites will be copied into:

  AiPersona.where(id: [-18, -19, -20, -21, -22, -23, -24, -25, -26]).default_llm_id

• SiteSetting.ai_summarization_model's existing value on sites will be copied into

  AiPersona.where(id: [-11, -12]).default_llm_id

Eventually, we will follow-up in a separate commit to remove the following settings:

SiteSetting.ai_helper_model
SiteSetting.ai_helper_image_caption_model
SiteSetting.ai_summarization_model
SiteSetting.ai_embeddings_semantic_search_hyde_model

By default SiteSetting.ai_default_llm_model will have default: "" in it's yaml file, however, this update also adds a migration which will ensure that the last created LLM model is used to fill the setting.

📸 Screenshots

[romanrizzi]

Should we do ai_translation_model too?