* Some more e-mail notification fixes

A few more small e-mail notification fixes/changes

* Style footer of notification email to be smaller
* Include text for when pull request is merged
* Don't include original body of issue or PR when merging/closing by
setting issue.Content to "" in these cases

* Set Re: prefix and meessage-ID headers based on actName instead of checking for a
comment. This fixes a bug where certain actions that didn't have a
comment were setting Message-ID instead of In-Reply-To which caused some
mail programs not to show those messages as they would have had the same
Message-ID as a previous message. Also fixes the case where a final
comment and closing message would have been displayed out of order if
you didn't have a copy of the original issue/pr cretion message.

* Update other template footers for consistency