Update the commit template to include Issue

Summary:
IN ADR #237, we decided to include a link to the issue in the template.  As such, the template needs to be updated and updated for everyone.

Issue: https://khanacademy.atlassian.net/browse/ADR-237

Test Plan:
1. Navigate to khan/devtools/khan-dotfiles
2. Update the repo (git p)
3. In webapp, create a new commit.
4. Verify the Issue field is presented to you

Reviewers: kevindangoor, csilvers

Reviewed By: kevindangoor, csilvers

Subscribers: csilvers

Differential Revision: https://phabricator.khanacademy.org/D57853

.git_template/commit_template

@@ -1,9 +1,15 @@
-<one-line summary, followed by blank line and full summary>
+<one-line summary>
 
-Test Plan:
+<full summary>
+
+Issue: <url or "none">
+
+Test plan:
 <see below>
 
 # --
+# For 'Issue:', A link (JIRA, Slack, etc) to the issue that
+#   drives this change or "none" if there isn't a link
 # For 'Test Plan:', list the commands you ran or process you
 #   followed to test this change.
 # If this commit is adding to an existing review, delete the