Add preference to disable emoji autocomplete picker

What does this MR do and why?

Add preference to disable emoji autocomplete picker; defaults to on!

Closes Add a Behavior preference to disable the emoji ... (#20231).

Screenshots or screen recordings

	Before / After with preference on (default)	After with preference off
Setting (After only)
Plain-text editor
Rich-text editor

How to set up and validate locally

1. Test the plain- and rich-text editors' emoji autocomplete function in e.g. a work item comment. Note they behave per the first column.
2. Check out this branch and run migrations.
3. View your user preferences; the setting checkbox should be checked per the first column.
4. Test the emoji autocomplete function again; the behaviour should be unchanged.
5. Uncheck the setting and save your user preferences.
6. Test the emoji autocomplete function again (be sure to refresh the page if you're using a left-over tab from before changing your user preferences!). It should behave per the second column.

MR acceptance checklist

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