Join form needs an `action` value so it works anywhere it's shown.

Since the join button can be displayed anywhere a WordPress shortcode
can be used, it may appear on page listings as well as single posts.
When that was the case prior to the patch, the user would not get
added to the project because they would get redirect to the listing
page instead of the form processing code (which is also the single
post, though this should probably get refactored sometime later.)


git-svn-id: https://plugins.svn.wordpress.org/volunteer-project-manager/trunk@871344 b8457f37-d9ea-0310-8a92-e5e31aec5664