Implement access control of project through project_id and channels

[mickmister]

We plan on using channel memberships to map users to project visibility. For this, we'll need to make channel membership lookups be efficient. We can have them in the same database, for performance and ease of development. We can put the Music Sniper schema on top of Mattermost in the same database, and use SQL when needed to bulk look up channel memberships and join objects with memberships.

This will also involve relating workspaces to MM teams