Allow users to reject their own watch/blacklist PRs (v2: Now with cleaner commit history!)

[Spevacus]

This is #11285 but with a less messy commit history (and a small regex adjustment).

I'd have done rebase-foo on THAT PR instead, but my Git wizardry is... Novitiate. That's a word, right?

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Implements feature request #8642

This PR allows users to reject/close (including -force) their own auto-created watch and blacklist PRs.

This was tested in this chatroom (where I removed SD ignoring its own messages when considering them for commands and some other changes, the full diff of which is available here where I mistakenly committed them to this branch and had to remove them).

One small change I made was making the payload argument for get_pull_request an optional arg with a blank string as its default. This change has no effect elsewhere in the project as get_pull_request isn't used anywhere aside from in this PR's changes, and you don't need to supply a payload when performing a GET request for a PR anyway.

I also took the liberty to remove the "so the user can learn from their mistakes" content from the required-rejection CmdException. I have a couple of reasons for this removal:

• This wording makes no sense if the user is rejecting their own PR
• We frequently reject duplicate PRs. In such a case, there is no mistake being made.

Known limitations:

This PR operates off of the chat messager's chat id, and so does submitting a watch PR. This can be different across chat servers, such as if a user submits a watch PR in SO chat and attempts to self-close it in Charcoal HQ. This will yield a "You need blacklist manager privileges to reject pull requests that aren't created by you." error.

Given that 95% of all watch/blacklist PRs occur in Charcoal HQ, I consider this an acceptable limitation.

[gparyani]

FYI: this was already requested at #1639 before and was declined. stroopwafels - I confused that for this

[tripleee]

Tested and approved. https://chat.stackexchange.com/rooms/154945/room-for-tripleee-and-ad-oisee

[tripleee]

Sorry for the really long time this took. I'll try to plow through the backlog of pending PRs over the next few weeks.