PR's should provide motivation & context first

It's often necessary to understand why a change is made, before
understanding the exact changes that are made. Context provides
background, which by definition is necessary to understand prior to the
substance of the Pull Request.

Change the PR template to request "Motivation and Context" first, before
"Description".

Reviewed-by: Giuseppe Di Natale <guss80@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matthew Ahrens <mahrens@delphix.com>
Closes #7737