Option to globally disable "atlantis apply"

[gezb]

Would you accept a PR that adds a new flag to the server that completely disables the atlantis apply command?

My use case here is we already have an established pipeline that does the terraform apply once a PR has been merged to master, we just want to use Atlantis to give us the preview of what a given PR will do.

My thinking is to add another check in command_runner.go - RunCommentCommand() that does a similar check as is done for DisableApplyAll

[mwarkentin]

This issue should be able to be closed now?

[gezb]

fixed by #1230