Add Channel Access Control plugin for Mattermost

[gluckdev]

Created a complete Mattermost plugin that restricts channel posting access based on user whitelists.

Features:

• Server-side hook (MessageWillBePosted) to intercept and validate messages
• Admin console UI with custom React component for managing access rules
• Visual interface to add/remove users from channel whitelists
• Configurable block message and admin bypass option
• Support for multiple channels with individual user whitelists

Technical implementation:

• Server component in Go with proper configuration management
• Webapp component in React with visual rule management
• Complete build system with Makefile
• Comprehensive documentation in Russian and English
• Proper plugin manifest with settings schema

Files included:

• plugin.json: Plugin manifest and settings schema
• server/plugin.go: Main plugin logic with hooks
• webapp/src/: React components for admin console
• Makefile: Build and deployment scripts
• README.md: Complete documentation and usage guide

Summary

Ticket Link

[mattermost-build]

Hello @mikhailartamonov,

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