Distinguish matching and non-matching annotations and replies when the sidebar is filtered

When a search filter is applied in the sidebar (for example filtered to show just one annotation after following a bouncer link; to show just one group; just one user; to match an arbitrary keyword search, ...) add a clear visual distinction between the annotations and replies that match the filer vs other annotations and replies that're being shown because they're in an expanded part of the same thread as one of the matching annotations.

While this will be a general improvement to client UX, it'll be particularly important when a teacher is using the client to grade a student's annotations in an LMS, and the client is filtered to match the annotations and replies of the student being graded.