Allow pastes to be activated/archived via Conduit

Summary: Ref T9964. Add a setIsConduitOnly() method so we can mark a field as API-only.

Test Plan:

• Created and edited pastes via web UI (no status field).
• Adjusted status via web UI action.
• Adjusted status via Conduit API.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9964

Differential Revision: https://secure.phabricator.com/D14788