Rewrite the access control system to only include the moderators

[ailisp]

The legacy contract had posts and label-based access control system. In the new contract, we'll deprecate posts with near social posts and the proposal workflow, which permissions are managed by Near Social and proposed SputnikDAO contract. Community admins are stored in community metadata. Therefore we don't need most of current permission system. We only need to add, remove and tell an account is a moderator.

Acceptance criteria

• Clean up access control code from the main contract
• Implement has_moderator
• Implement add_moderator and remove_moderator, only allowed by account owner (devhub.near)