Mattermost and Slack integration

Issues in priority order

• 9.0 #23964 (closed) When creating GitLab group, create Mattermost team
• #26432 (moved) Jump to Mattermost channel from GitLab issue
• #25269 (moved) Allow Mattermost team creation when enabling Mattermost Command
• #27514 (closed) Create Mattermost channel during GitLab project creation