This PR fixes #12803 by supporting short multiline messages in SD. For example, it makes `!!/location<NEWLINE>Hello` work.

[elsisi33]

This PR fixes #12803 by supporting short multiline messages in SD. For example, it makes !!/location<NEWLINE>Hello work.

Here's a before and after.

The issue was short multiline messages start with <div class='full'>, which SD doesn't know to remove. As a result, it doesn't see the message as starting with either !!/ or sdc and ignores the messages. There's multiple ways to handle the ending </div> (see this Stack Overflow question). I opted to use a bool variable, but I'm happy to go with something else too.

Originally posted by @CoconutMacaroon in Charcoal-SE/SmokeDetector#13797