Fix user autocompleting
This rewrites quite a lot of QueryMatcher.
* Remove FuzzyMatcher which was a whole file of commented out code
that just deferred to QueryMatcher
* Simplify & remove some cruft from QueryMatcher, eg. most of the
KeyMap stuff was completely unused.
* Don't rely on object iteration order, which fixes a bug where
users whose display names were entirely numeric would always
appear first...
* Add options.funcs to QueryMatcher to allow for indexing by things
other than keys on the objects
* Use above to index users by username minus the leading '@'
* Don't include the '@' in the query when autocomple is triggered
by typing '@'.
Fixes https://github.com/vector-im/riot-web/issues/6782
