app_confbridge: Add ability to get the muted conference state.

* Added CONFBRIDGE_INFO(muted,) for querying the muted conference state.

* Added Muted header to AMI ConfbridgeListRooms action response list
events to indicate the muted conference state.

* Added Muted column to CLI "confbridge list" output to indicate the muted
conference state and made the locked column a yes/no value instead of a
locked/unlocked value.

ASTERISK-20987
Reported by: hristo

Change-Id: I4076bd8ea1c23a3afd4f5833e9291b49a0c448b1