Warn user about @ mentioning the addressee in 1:1 PMs

In some orgs using other chat tools (e.g. Slack), it is common to start 1:1 PMs by @ mentioning the user being addressed (e.g. "Hi @kelly!"). Users with this habit may do so on Zulip as well. This results in potentially unexpected behavior, where the message is highlighted and shows up in @ mentions in addition to PMs.

In order to address this, we should warn users about the impact of using an @ mention in that context. E.g. something like: “Mentioning <addressee> will highlight this message. Did you mean to use a silent mention (@_<addressee>) instead?”

This warning should apply to 1:1 PMs only, as @ mentions are more likely to be intentional in group PMs.

Note that other compose box warnings can be found in /static/js/compose.js, e.g. validate_stream_message_mentions.

[asah]

@KarishmaVanwari PR is waiting for final review.

[PVBharadwaj]

@zulipbot unclaim