qualify list-id when generating tags

Using an unqualified list-id as a tag name allows anyone sending you
mail to set arbitrary tags on messages.  This patch puts all list tags
in a 'list/' namespace.

For example, I could send you a message containing:

    List-Id: This is very important <urgent.example.com>

And the existing `ListMailsFilter` would happily add the `urgent` tag to this message.