Autocomplete is case sensitive

[karger]

Username @lea has demonstrated a bug in the autocomplete for tagging users in a chat---it's case sensitive, so typing @l doesn't include @lea in the autocomplete list.

Also, as autocomplete lists get larger, I think we might want to be a bit pickier about what gets listed in autocomplete---e.g., maybe not everything with an L in it should should up when I type @l

[amyxzhang]

along those notes, typing a @ and then a space should dismiss the autocomplete popup. also in chat there's not enough room for the autocomplete menu.

[kelly-shen]

autocomplete is now insensitive.

[joshblum]

@kelly-shen can you submit a pull request with the code for this?