NethVoice: make easier manage a user

[Amygos]

Add a more convenient way to change a user's configuration:

• if I have to change a display name, the change does not reach the display name of his phone (the extensions in FreePBX are not updated)
• if I have to replace a user with another (keeping the main extension unchanged and the associated phones) it would be convenient to do so without having to reconfigure it from scratch, especially regarding the assignment of devices
• if I have to delete a user make sure that his extensions are deleted, and his physical devices are freed

[andrea-marchionni]

also think of a way to bring the change into the configuration of the phones' BLF