Post Detail Action Buttons - Fixes T344

Summary:
* adds "next" & "previous" buttons

* fixes post detail action button layout

Test Plan:
* load post detail page
* check for "Prev" & "Next" post nav buttons
* resize browser for responsive views

Reviewers: shadowhand, rjmackay

Reviewed By: rjmackay

Subscribers: digitalafrican, dkobia

Maniphest Tasks: T344

Differential Revision: https://phabricator.ushahidi.com/D186