assistant: Add action footer and refine slash command popover (#16360)

- [x] Put the slash command popover on the footer
- [x] Refine the popover (change it to a picker)
- [x] Add more options dropdown on the assistant's toolbar
- [x] Add quote selection button on the footer

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Release Notes:

- N/A

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Co-authored-by: Piotr Osiewicz <24362066+osiewicz@users.noreply.github.com>
Co-authored-by: Nate Butler <iamnbutler@gmail.com>
Co-authored-by: Kirill Bulatov <mail4score@gmail.com>