Simplify template argument names

Now templates are easier to write than ever!

Instead of writing ${comment_creator_url}, you now only have to write
${user_url}, and instead of ${issue_body}, just plain old ${body} will
suffice.

Note: This will break existing templates. :( I could have included reverse-compatibility, but my templates look so sexy, I doubt many people have created their own, and instead just use the default templates.