SignIn Form

Summary:
* cleans up signin form

* double spaces between classes for easy viewing/scanning
  * this is a standard I have adopted and helps with large html files

* adds signin specific styles and markup

Test Plan:
* load signin view
* confirm form is wrapped with white background and centered
* test responsive views

Reviewers: rjmackay, shadowhand

Reviewed By: shadowhand

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

Differential Revision: https://phabricator.ushahidi.com/D374