Fix templates to properly wrap the whole body with quotes

[larkox]

Several templates are formed like this:

>{{.GetPullRequest.GetBody | trimBody | replaceAllGitHubUsernames}}

If the body had several line breaks, this may format the body as:

> A

B

This will make the first line be quoted but the second not. In order to avoid this, every line break should have a >. i.e.:

> A
>
> B

This changes should be covered by a lot of unit tests to ensure no edge cases are missed.