Bug 1032694 - Elements misaligned on new task creation page

I updated the styling of the form to address the issues in the bug, and to use the sandstone style appropriately as per http://ossreleasefeed.github.io/Sandstone/

@glogiotatidis r?
@ossreleasefeed mind taking a look at the finished product on dev [1]?

[1] https://oneanddone-dev.paas.allizom.org/en-US/tasks/edit/4/