lightbox: Show user's avatar in the lightbox app bar

In the lightbox, the RN app shows the message author's avatar along with their name in the app bar; we should do the same in the Flutter app. Like in the RN app, probably the avatar should go beside the name/date text, in the "start" direction (left in LTR).

Perhaps it should have a presence indicator (?) but that could be done in a later iteration.