Reject multiline chat messages attempting to watch/blacklist keywords

[CoconutMacaroon]

Fixes #12732 by telling the user

The watchlist/blacklist cannot be modified using multiline chat messages. If your desired change exceeds the length of a standard chat message, consider using Git to make the change (or ping someone who can).

I tested this in a separate chatroom here, and testing a multiline watch that would exceed the normal length limit produced an error.

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~~However, a short multiline watch (chat link) produced no response; however, doing so also didn't appear to break anything. Of course, silently ignoring a command isn't exactly ideal either, so that needs either:~~

• ~~Someone to say "Eh, good enough"~~
• ~~Or I should figure out why that happens and fix it~~

See this section of transcript regarding the crossed-out text above. I didn't have time to finish investigating that, but the short version is this:

It wasn't caused by this change (it's reproducible with unmodified SmokeDetector). I suspect it's related to these lines of code (specifics here), but I haven't finished investigating it and haven't confirmed that's the root cause.