Refactoring main markup

Summary:
* renames `.main` to `.main-content`

* refactors signin markup and styles

Test Plan:
* make sure app layout is not broken
* check markup

Reviewers: rjmackay, shadowhand

Reviewed By: shadowhand

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

Differential Revision: https://phabricator.ushahidi.com/D397