Disable multiple deploy submissions on deploy form

This patch disables the deploy button on the deploy form after it has
been clicked once.  This prevents multiple submits being sent by the
same form.  The patch also changes the deploy route to POST to avoid the
user accidently submitting deploy when navigating to /deploy.
Merge pull request #123 from mtaylor/disable_multiple_deploy_submits

Disable multiple deploy submissions on deploy form