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

src/autocomplete/UserProvider.js

@@ -45,7 +45,8 @@ export default class UserProvider extends AutocompleteProvider {
         super(USER_REGEX, FORCED_USER_REGEX);
         this.room = room;
         this.matcher = new QueryMatcher([], {
-            keys: ['name', 'userId'],
+            keys: ['name'],
+            funcs: [obj => obj.userId.slice(1)], // index by user id minus the leading '@'
             shouldMatchPrefix: true,
             shouldMatchWordsOnly: false,
         });