Put best autocomplete matches at bottom of list, not top

That way they're in easy reach. In particular when using a phone with one's thumbs, the top of the screen can often be significantly harder to reach.

Because the query is at the bottom of the screen (in the compose box) and the list of autocomplete options opens upward from there, putting the best matches at the bottom is also logical because it puts them right next to the query (the one they're the best match for).

This applies to both ComposeAutocomplete (for autocompleting @-mentions, emoji, and so on from the content input) and TopicAutocomplete (for autocompleting topics from the topic input).