Settings

Summary: * adds header mixin to site settings

Test Plan: * make sure site setting has title header

Reviewers: rjmackay, shadowhand

Reviewed By: shadowhand

Subscribers: spaudanjo, sebm90, digitalafrican, eyedol, lkamau, srutto, dkobia

Differential Revision: https://phabricator.ushahidi.com/D411