Add shelve_personalization method to Sympa::List

[racke]

We are using blocks like the following in various parts of the code:

        # - Shelve personalization.
        if (    'on' eq ($list->{'admin'}{'personalization_feature'} || 'off')
            and 'none' ne
            ($list->{'admin'}{'personalization'}{'web_apply_on'} || 'none')) {
            $u_message->{shelved}{merge} =
                $list->{'admin'}{'personalization'}{'web_apply_on'};
        }

These blocks should be replaced by a call to a new method shelve_personalization in the list class.
It will take type as parameter (web or mail), shelves when appropriate and returns a true value if the message is going to be personalized.