matrix-synapse: STUN/TURN configuration

• Matrix Synapse picks up the locally installed coturn server during its installation.
• Admin can override the STUN/TURN server settings for Matrix Synapse.
• Coturn auto-configures apps using its component on its installation, if they're not already configured.
• Coturn reacts to domain name changes and auto-configures apps
• Update developer documentation

Closes #1122 (closed)