apps/app_voicemail.c and main/say.c: Add support for Icelandic language

Icelandic has some weird grammar rules when dealing with dates and
numbers. There are different genders used depending on which number
you're dealing with, and only a handful of numbers do change depending
on the gender. There is also an implied gender in several cases.

This patch was originally written for asterisk 1.6, and has been in use
for several years without crashes. I cleaned it up a bit and rewrote
what was necessary for Asterisk 13.

The functions were copied from other similar languages and modified
where appropriate. If i recall correctly, the German and Danish
functions were used as a base.

ASTERISK-26087
Reported by: Örn Arnarson
Tested by: Örn Arnarson

Change-Id: Ib7d8bd7b0fede5767921ed821315b5b508c0e665