mailman: use a new default theme (prettier mailman)

Use a new design based on one used on a few lists. This is the general
version instead of the a specific one. Should be good for all lists.
Adds a Wikimedia logo which might be the only 'non-general' thing
but it good. The theme must be translated for all lanuages via adding
a templates/$lang/listinfo.html file otherwise it will display in English.
A Meta-Wiki page already exists to translate this to different languages
so it should not be long for this to meet the needs of the community.

This will *not* overwrite the theme used by lists if they have
customised it. It will only change the theme for lists which still use
the default one aka. 'never edited the file in mailman admin', tested on
mailman labs project.

Bug: 61283
Change-Id: Ie4bf9a3b4610476cccc9f67e6f2701fb0702fefc