Set message and group used on [[Special:SpecialPages]]

for [[Special:Campaigns]]. Currently it is listed under "Other special
pages" as "&lt;campaigns&gt;" (double-escaped), which looks weird.

Also the use of "Special:Campaigns" might be worth re-considering as
there will be possibly more campaigns, not only upload campaigns. But
this is not addressed by this patch.

Change-Id: I600fce1c320f51782a0cc91bcead9997e3acc611
Merge "Set message and group used on [[Special:SpecialPages]]"