Add Post to Set

Summary:
* adds controller and route

* adds anchor href

* adds template and sass file

* adds header mixin

* registers new pagination partial

* adds disable/enable save button
    * this does not work fully, I left the code in to get feedback. There are several ways to accomplish this, but I am trying `ng-show` based off a `.hasClass` boolean.

Test Plan:
* load add
* click "set" in post actions dropdown button
* select and deselect Sets

Reviewers: rjmackay, shadowhand

Reviewed By: shadowhand

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

Differential Revision: https://phabricator.ushahidi.com/D309