app_sla: Migrate SLA applications out of app_meetme.

This removes the dependency of the SLAStation and SLATrunk
applications on app_meetme, in anticipation of the imminent
removal of the deprecated app_meetme module.

The user interface for the SLA applications is exactly the
same, and in theory, users should not notice a difference.
However, the SLA applications now use ConfBridge under the
hood, rather than MeetMe, and they are now contained within
their own module.

Resolves: #50
ASTERISK-30309

UpgradeNote: The SLAStation and SLATrunk applications have been moved
from app_meetme to app_sla. If you are using these applications and have
autoload=no, you will need to explicitly load this module in modules.conf.

Author: InterLinked1