Added onsubmit in cancel form

A confirmation dialog was added to the cancel button to prevent immediate cancellation, as the cancel button is located right after the log, which could lead to unintended cancellations by the user.

scrapyd/website.py

@@ -132,7 +132,7 @@ def __init__(self, root, local_items):
         self.prefix_header = root.prefix_header
 
     cancel_button = """
-    <form method="post" action="{base_path}/cancel.json">
+    <form method="post" onsubmit="return confirm('Are you sure?');" action="{base_path}/cancel.json">
     <input type="hidden" name="project" value="{project}"/>
     <input type="hidden" name="job" value="{jobid}"/>
     <input type="submit" style="float: left;" value="Cancel"/>