Bridging: introduce "invisible" bridges.

Invisible bridges function the same as normal bridges, but they have the
following restrictions:

* They never show up in CLI, AMI, or ARI queries.
* They do not have Stasis messages published about them.

Invisible bridges' main use is for when use of the bridging system is
desired, but the bridge should not be known to users of the Asterisk
system.

ASTERISK-25925

Change-Id: I804a209d3181d7c54e3d61a60eb462e7ce0e3670