post detail fixes

Summary:
* fixes post detail width

* adds background color to meta info for better visual separation

Test Plan:
load post detail page
check the width of meta info and main content on large screens

Reviewers: brandonrosage, shadowhand

Reviewed By: shadowhand

Subscribers: spaudanjo, sebm90, digitalafrican, srutto, dkobia, rjmackay

Differential Revision: https://phabricator.ushahidi.com/D532