Allow to modify the system prompt after the playground interactions started

[slemeur]

After #617, the user will not be able to modify the system prompt, once the playground interactions started.

In this issue, we would to better handle the situation and let the user modify it, but also understand the impact of that change.
In order to allow the comparison of the behavior of the model with a different system prompt, the user should still be able to see the previous interactions (and eventually continue testing with more interactions, with the different system prompts).

Proposal:
When the user changes the system prompt after having started interacting, we should bring the user into a new playground environment with the new system prompt.

We would display a message asking the user what the desired "re-play" stated is.
Two options to be proposed to the end-user:

• Does the user wants to replay all the messages sent?
• Or does the user prefer to start from scratch?

[axel7083]

I would argue that re-playing all the conversation does not really make sense, I would propose that only the first message of the user would be replay, as the responses following it depends on the conversation, which do not have the same history after changing the system prompt.