Add User-Based Filtering for MR Comment Webhook Notifications in Mattermost/Slack Integration

Summary

Allow Mattermost/Slack webhook notifications to exclude comments from specific users (e.g., @GitLabDuo) to reduce noise from automated code review comments.

Problem to Solve

When GitLab MR Comment webhooks are enabled, automated reviewers like @GitLabDuo generate many comments, causing excessive Mattermost/Slack notifications. This makes it difficult for teams to notice important human-written reviews.

Proposal

Add an option in Mattermost/Slack integration settings to exclude notifications from specific users. For example:

Exclude comments from: @GitLabDuo

This could be a text field, multi-select box, or list of bot accounts.

Intended Users

• Teams using GitLab ↔️ Mattermost integration
• Teams using AI/bot reviewers generating many automated comments

Benefits

• Reduces notification noise from automated systems
• Keeps important human review notifications visible
• Improves focus and signal-to-noise ratio in review channels