Fix cancel button size in modal dialogs

The cancel button in a modal dialog is being modified by different
css classes than the other buttons "submit/ok/etc.." This is due
to an explicit .close button class in _close.scss. Adjusting that
would have wider ranging ramifications than just the modal dialog
close/cancel buttons, so instead I modified the existing font
size to make it match the submit button, and explicitly set the
line-height to the default (.close has its own line height setting
that doesn't match whats in .btn).

Change-Id: I9f254df8c922105b9fcc9a8beaced639bb2b1d4a
Closes-bug:1359871
Merge "Fix cancel button size in modal dialogs"