Styling changes to padding, margin, color, and mw-ui-quiet

Requested at https://trello.com/c/MaqpJE5f/405-create-pre-edit-anonymous-editor-signup-invitation-guided-tour ,
except mw-ui-quiet which is based on a mockup

* Float quiet buttons to the left, and set margin-left and horizontal
  padding to 0
* Lower whitespace above guider title
* Add more whitespace between description and buttons
* Set description color to #252525

Change-Id: If8e27c85fdda9a828c79edbfc298bc3d66008e6b