This diff adds new SearchIndex selector for user mentioning. We need to use SentencePrefixSearchIndex rather than SearchIndex, because SearchIndex catches keywords rather than whole text prefix, so when user would type whole username and then input a space, the tooltip would never close.
Depends on D8900.
Does this apply to all targets now? Are we sure we want to apply it to all targets? Is it possibly that we're inadvertedly applying it to some targets we haven't considered? (Targets that aren't the Comm app, or the notification service extension)