timezone: better timezone integration, fixes for default timezone selection.
Added option to set a fixed timezone for a site.
Added #user_context{} notification, folds with the context to set all user-preferences in a context.

(cherry picked from commit 9629469f0afe62b7f3af6aa5f7a3c7679da39a27)

Conflicts:
	modules/mod_l10n/templates/_admin_l10n_config.tpl
mod_l10n: merge template change from master.