gigaset N870: handle Handset= in User-Agent

Reviewed-by: https://github.com/apps/wazo-production-sf

WAZO-2371: define codecs

Reviewed-by: https://github.com/apps/wazo-production-sf

Why: otherwise the phone sends too much information in the SDP
negociation.

WAZO-2373: add Yealink firmware v86

Reviewed-by: https://github.com/apps/wazo-production-sf

Gigaset: fix N870 documentation link

Reviewed-by: https://github.com/sduthil

WAZO-2406 / WP-360: add Gigaset N870

Reviewed-by: https://github.com/apps/wazo-production-sf

WAZO-2383: fix Yealink v81 firmware links

Reviewed-by: https://github.com/apps/wazo-production-sf

reason: the handsets use this frequency band only.

reason: N870 does not restrict the provd version to >= 0.2 like the N720, so
we need to make it compatible with previous versions.

reason: if the extension is shorter than 4 characters, we want a 4 character code anyway.

reason: v2.17.2 supports fixed handset numbering.