Merge pull request #147 from BnLG/n870

Gigaset N870 enhancements

Hello,
This PR brings enhancements for Gigaset N870 devices.
It's a whole copy of N870-83.v2.39.0 plugin, with following differences :

fix a bug with sip_subscribe_mwi handling preventing provd from building the template
fix of the _UA_REGEX for devices with old firmware (for ex 2.39) which get their new firmware sending their UA between plain double quotes...
update to last 2.48 firmware version (basic functions tested, working)
allow mobile devices firmware update
small cosmetic update on display name, to mimic other Gigaset devices, such as Gigaset N720
proper SIP TLS / SRTP handling

Unfortunately though, I've encountered a bug between N870 and Asterisk using SIP TLS, where calls are terminated as soon as they're put on hold (which is rather annoying).
I've then disabled SIP TLS support for now, I'll document bug report here for ref, and I'll bring an update of the plugin once solved.
Thank you 👍

Reviewed-by: Alexandre Fournier <None>