RFR: pr: add /backport commit command [v2]

[Robin Westberg]

On Thu, 5 Nov 2020 14:28:25 GMT, Robin Westberg <rwestberg at openjdk.org> wrote:

>> Erik Helin has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Wording
>
> bots/pr/src/main/java/org/openjdk/skara/bots/pr/BackportCommand.java line 64:
> 
>> 62:         var username = command.user().username();
>> 63:         if (censusInstance.contributor(command.user()).isEmpty()) {
>> 64:             reply.println("@" + username + " only OpenJDK [contributor](https://openjdk.java.net/bylaws#contributor) can use the `/backport` command");
> 
> The command handler will prefix the reply with `@command-issuer` so should not need to do that manually.

Scratch that, it's only done for regular pull requests commands.. Something to fix another day.

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PR: https://git.openjdk.java.net/skara/pull/945