This PR introduces a modern floatkit-based autocomplete system for the core composer (both rich text / markdown modes), intended to replace the legacy jQuery-based implementation. This will be enabled via a site setting for now. All tests that cover the legacy implementation are duplicated with the site setting enabled to ensure they keep functional parity.

What's changed:

• The autocomplete menu remains open between searches while typing within a search term, instead of closing and reopening (this looks like the menu flickering, especially if the searches are quickly resolving).
• Flip behaviour now works (the autocomplete menu should never overlap with the header, and will appear below the cursor if there's not enough space to appear fully in the viewport)
• On any mouse-down event outside the menu, the menu will immediately close (previously, it stayed open during the grippie drag up/down of the composer drawer, and closes on mouse-up)
• Preserves exact CSS structure and selectors for existing themes/plugins
• Better use of native browser APIs
  • scrollIntoView API is used for handling scroll within the hashtag autocomplete menu instead of manual calculation
  • requestAnimationFrame API is used to better time the opening of the autocomplete menu with repaint during pasting of autocompletable terms
• position: absolute CSS was removed - this didn't seem to affect the old autocomplete during testing, and keeping it broke positioning for the Floatkit-based autocomplete

What's the same:

• All templates specific to the different types of autocomplete (user & group / hashtag / emoji) remain exactly the same
• we update the selected class that's used to highlight the item in the autocomplete menu while navigating it via keyboard the same way we do in the old autocomplete - it's fairly imperative, but allows us to avoid a deeper refactor (including an entirely new set of templates)