Add a command to allow rejecting a pull request as a duplicate

[gparyani]

Often, pull requests have to be closed as duplicates (such as if a privileged user watches or blacklists a keyword that has a pending PR by a non-privileged user, so that PR has to be procedurally rejected). Per policy, these are labeled with the "type: duplicate" label so that users searching through one's PR history can tell between PRs that are procedurally rejected for this reason and those that are actually rejected.

This code change adds the !!/reject-duplicate command, so that users who procedurally reject duplicate PRs don't have to go to the issue after the fact and apply the label manually: it can be done with one command.

[makyen]

Has this code been tested by running it on a test SD instance in a chat room against a fork of the repository? If so, please provide a link to the chat room/start of test messages.

[gparyani]

@makyen No, I haven't done that. Is doing so required for this to be merged?

[makyen]

Given that this is doing something on GitHub, which isn't necessarily easy to recover from something going drastically wrong and/or possibly causing permanent loss of some data, yes, there needs to be testing prior to merging/deploying. That doesn't mean that you have to do it, just that it needs to be done.