Allow one to share system prompts (assistants)

[tarasglek]

We need a better way to share prompts.

1. I need to share my prompts among my devices
2. I need to share prompts with other chatcraft users.

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1. One lazy way to do this is to implement it similar to how we did function calling by storing them in github gists, dropping a link to gist into system prompt and replacing contents of system prompt with contents of link dynamically.

2. Deepset does it like this, it's kinda lame https://github.com/deepset-ai/prompthub/