Voicemail: Add the possibility to define which timezone to use when playing vocal messages

[Pontax]

Vocal messages dates are stored in UTC format. When the voicemail plays a message and its date, it may use a specific timezone.

2 possibilities :

• use the server's timezone
• add a configuration variable

[shimaore]

This is implemented in f77acbf51391156de734a6a58746d99fb309337d et seq.