Enable a signal for help command in community MR

What does this MR do?

Add reaction to help request:

• Post a slack message to #mr-coaching to request for help
• Reply to MR author with a message

This reaction is applicable to the following:

• notes on MR in gitlab-org starting with @gitlab-bot help
• user is not rate limited (max 1 per day per MR)
• user must be author of MR

considerations

Considerations listed in #54 (closed):

• Have a user age requirement to trigger this automation

🚫 GitLab doesn't store age.

• Have an idle duration requirement that needs to be met

⚠ currently using default rate limit of 60 per hour. May need to customize rate limit to 1 per hour or day, considering this reaction posts to Slack and we should reduce noise in Slack.

• Respond only to first help request in the merge request

⚠ not implemented yet, we can consider this in subsequent iteration

• Note author must be MR author, so a stranger cannot spam others' merge requests.

✅

Example

Example slack message:

Closes #54 (closed)