[Feature] Show user note when mentioning a user

This is a cool idea:

> Neil Brown
> @neil@mastodon.neilzone.co.uk
>
> It's a cool feature of Mastodon that you can leave a free-text note, visible only to you (and your admins etc.) against someone's profile, whether you follow them or not.
>
> What would be especially cool is if you could have that note pop-up when you start a toot mentioning that person, so that the note is foregrounded at the right time.
>
> e.g. "Neil doesn't like jokes" popping up if you mention Neil, so that it is at the forefront of your mind before you post a joke mentioning Neil.

https://mastodon.neilzone.co.uk/@neil/111539713466831418