Post Detail

Summary:
* adds post detail markup and styles
    * markup in `detail.html`
    * sass in `post-detail.scss` & `sidebar--post-detail`

* adjusts @import order and files

* other small html & sass changes

Test Plan:
* load post detail
* check markup
* resize browser for responsive views

Reviewers: rjmackay, shadowhand

Reviewed By: shadowhand

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

Differential Revision: https://phabricator.ushahidi.com/D295