* 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

This commit improves templates readability, since all of them use consistent
indent with all template command blocks indented too.

1. Indents both HTML containers such as <div>, <p> and Go HTML template blocks
   such as {{if}} {{with}}

2. Cleans all trailing white-space

3. Adds trailing last line-break to each file

Removed trailing whitespaces with

```sh
for file in `grep -r " $" * | cut -d : -f 1 | sort | uniq | grep -v " "`; do sed -i 's/[ \t]*$//' $file; done
```

and some handwork.