Improvements in display of footer autocomplete options

[neiljp]

This needs further discussion, but could include:

• making it clearer where each matching entry starts and stops (spacing? separators? grid layout?)
• clarifying which entries are groups vs users (maybe also vs streams vs emoji, but @ can particularly match multiple in one autocomplete list)
• improving handling for when there are more than 10 matches (support scrolling? suggest options beyond just showing more?)
• showing hints for what keys to use?
• showing a prefix like matches: or similar?