Manage Users

Summary:
* adds Tools/Manage Users in workspace menu
    * adds route
    * adds html accordion-group

* adds manage user
    * template
    * sass
    * user.js
    * user filter.js

* adds reusable list filter--alt

* adds reusable filter

### Sass Refactor/Restyle

* refactors header and header title
    * adds mixin
    * changes html, sass & js

* refactors list-view
* refactors search-bar

* Global Sass changes
    * forms
    * dropdown-button
    * `opacity` mixin
    * `%striped` @extend

Test Plan:
* click workspace/tools/manage users
* confirm page loads and "All" tab is active
* click UI elements for functionality
    * dropdown button
    * list filter
    * user filter
* adjust browser window for responsive views

Reviewers: rjmackay, shadowhand

Reviewed By: shadowhand

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

Differential Revision: https://phabricator.ushahidi.com/D343