Configuration option:  Ability to set ERT to be admin call only.

Admin request.

Default behavior is what we're used to, that two heads can call for the ERT
with the option turned on it removes the ability for players to call for an ERT and requires admin intervention.
Merge pull request #4505 from Ccomp5950/ert_admin_only

Configuration option:  Ability to set ERT to be admin call only.