Warn when mention all users in a group

What does this MR do and why?

Related #26889 (closed)

Show warning when mentioning @all in markdown fields. When we detect @all in the text field, make a request to the Preview endpoint to check how many people will be notified. If @all is removed we remove the warning.

This is meant as a sanity check against accidental @all mentions. Currently we show this in the preview tab, but if you don't click preview before posting a comment you never know!

Screenshots or screen recordings

How to set up and validate locally

1. Find a group with more than 10 members (or reduce referencedUsersThreshold in field.vue)
2. Go to an issue in that group
3. Type @all (not in quotes) in a markdown field
4. Realise your mistake, don't post the comment, and save thousands of notification emails

MR acceptance checklist

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