Improve the format and usability of dates in webconf invitations

The format now uses i18n.
The process of changing the dates is now more natural: duration is kept
in some cases, inputs are adjusted automatically, etc.
Add class and default method to show/get user timezones

Just wrapped the timezones around a class and a method to show timezones in
the views. Added the timezone to the webconf invitation form.
Fix the timezones for webconf invitations

Make it consistent. The user configures the invitation using his timezone,
and the emails are sent using the target user's timezone.
The .ics file was also fixed, now sending the dates always using UTC.
Add a title to webconf invitations

Used mostly for the .ics file. The title becomes the title of the event in
the .ics.