v0.21.0

• Camera filter on the review queue and the history, and a choice of how many history
  entries to load — suggested and prototyped by @WiredLife in #14, rebuilt here against
  the current UI.
• The filter only appears once there is more than one camera to choose between. With a
  single camera it would be a control that cannot change anything, and an empty dropdown
  invites the question of what it is for.
• On the review queue it filters clusters: a cluster survives when at least one face in it
  came from the selected camera, so a group is never silently cut in half.