Enhance styling for long message truncation to be reasonably stylable

[timabbott]

This is an iteration on #10887, and a somewhat low priority (since it's not a common situation in any case and probably a bit tricky to get right), but it would be good to make it look nicer.

One option is to (1) have it add somewhat more special syntax (e.g. <message_truncated>), rather than [message truncated] at the end of the message, and (2) have a markdown processor phase that runs super early (to avoid getting caught inside a code block or something) check if the message ends with that syntax; if so, it'd replace the syntax with an HTML span at the end of the message that we could later style appropriately.

Another option would be just to have the code that truncates it pass an attribute, e.g. message_truncated=True into the markdown processor; the problem with that is that we won't have the data stored in the database to find out, so we wouldn't be able to correctly rerender such messages.

[zulipbot]

Hello @zulip/server-markdown members, this issue was labeled with the "area: markdown" label, so you may want to check it out!

[gaharavara]

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