Allow users to modify user agent for Linkus

I found from release note that he latest version of firmware support to lock the extension to specific User Agent, I think if Linkus supports for modify user agent information, then we can lock specific Linkus(terminal) to specific extension. This will fix a lot security issue I believe. It would be better if we can setup the user agent on both the Linkus mobile App and on the PBX side. As normally, only administrator of the PBX care the security, so it would be better to configure this from PBX side.