adds "edit" to post-details ui

Summary:
once editing functionality is landed, clicking edit will allow inline editing or go to separate edit view (need final design decision on this)

Fixes T1190
Blocked by T1134 - editing functionality

Test Plan: * check layout

Reviewers: brandonrosage, spaudanjo, rjmackay, shadowhand

Reviewed By: shadowhand

Subscribers: sebm90, srutto, dkobia

Maniphest Tasks: T1190

Differential Revision: https://phabricator.ushahidi.com/D559