(EAI-626): Send slack message on user comment to feedback channel

[mongodben]

Jira: https://jira.mongodb.org/browse/EAI-626

Changes

• Send Slack message for user feedback
• Add slack token to staging environment (not in PR)

Notes

• We rerun the LLM-as-a-judge evals for this. This means that LLM-as-a-judge will rerun for all commented messages (and the subset of all messages that are also commented). This is not compute efficient but given the small number of feedback messages we get I think it's acceptable, in order to separate concerns.
• Example of message