Add admin toggle mute all and participant count menu options to app_confbridge

This patch adds two new menu features to app_confbridge, admin_toggle_menu_
participants and participant_count.  The admin action will globally mute /
unmute all non-admin participants on a converence, while the participant
count simply exposes the existing participant count function to the
conference bridge menu.

This also adds configuration options to change the sound played when the
conference is globally muted / unmuted, as well as the necessary config
hooks to place these functions in the DTMF menus.

(closes issue ASTERISK-18204)
Reported by: Kevin Reeves
Tested by: Matt Jordan
Patches:
  app_confbridge.c.patch.txt, conf_config_parser.c.patch.txt, 
  confbridge.h.patch.txt uploaded by Kevin Reeves (license 6281)

Review: https://reviewboard.asterisk.org/r/1518/



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@345560 65c4cc65-6c06-0410-ace0-fbb531ad65f3