Create Evidence::ActivatedLLMPrompt

[brendanshean]

WHAT

1. Create Evidence::ActivatedLLMPrompt model
2. Add favorite attribute to LLMPrompt

WHY

1. Curriculum wants to be able to see, not just which prompt is activated at a given time, but a history of activated prompts with their activation date
2. Curriculum want to be able to favorite the prompts

HOW

1. Add migration and model. Add a callback and validation that ensures a given prompt has at most one ActivatedLLMPrompt with active: true
2. Add migration that adds boolean column to LLMPrompt

Screenshots

(If applicable. Also, please censor any sensitive data)

Notion Card Links

https://www.notion.so/quill/Gen-AI-CMS-Project-2-Embed-Trial-Runner-in-CMS-61c994a106854512b449ccc82be573c2?pvs=6&utm_content=61c994a1-0685-4512-b449-ccc82be573c2&utm_campaign=T02HWALTZ&n=slack&n=slack_link_unfurl#12fd42e6f94180228c71c4a7882b6c55

What have you done to QA this feature?

(Provide enough detail that a reviewer could assess whether additional QA should be done. For larger projects, additionally use the Engineer Feature Testing Notion template. Review Guidelines if needed: https://www.notion.so/quill/Github-PR-QA-Guidelines-49e99fc965654ceeb8c6249bd9d181d7)

PR Checklist	Your Answer
Have you added and/or updated tests?	YES
Have you deployed to Staging?	YES
Self-Review: Have you done an initial self-review of the code below on Github?	YES

[emilia-friedberg]

Looks great! Thank you so much for your speedy work on this amidst a heavy support load.

[dandrabik]

Thanks for moving this along!