Fix config URL so that the phones don't get an invalid URL when $domain_name is unset. They would previously receive `/app/provision` with no domain/ip included, which makes the phone unable to provision again once they have been added to the Devices page without manually re-setting the config URL or factory defaulting the device.
The alternative solution is that the PHP code could always ensure that the $domain_name variable is set even if the device doesn't exist. My suggestion is to see if it can be set to the same domain/IP as the original provision request from this unconfigured device
* Added the ability to pick the SIP account for the first two slots on each handset
* Revert to clean base
* Configurable firmware versions for Grandstream Devices
* Cleaned up duplicate P codes in gxv3504 prov, and spacing elsewhere
Updated or added configurable firmware versions to the remaining Grandstream configs
* Documented how to use the firmware version feature
* Formatting fix in Readme
