Only show "Flag message" Action if user has permission

[bpollman]

🔗 Issue Link

🎯 Goal

User shouldn't see Flag Message action if they cant use it

🛠 Implementation

Simple capability check before adding Message action

🧪 Testing

Change user permissions and verify that the action can or cannot be seen

🎨 Changes

☑️ Checklist

• [x] I have signed the Stream CLA (required)
• [ ] Changelog is updated with client-facing changes
• [ ] New code is covered by unit tests
• [x] Affected documentation updated (docusaurus, tutorial, CMS (task created)