Mirador tweaks

Related to #440 (closed)

Related to #441 (closed)

Explanation of the Firefox fix: We already try multiple times to update the “view state” (zoom and pan parameters) because it’s hard to predict when Mirador will be ready with a viewport to zoom and pan. However, I apparently was only setting the canvas number once when the plugin first initialized. I still don’t know why this worked everywhere but Firefox, but my solution was to try to push set the canvas number every time we try to push the view state. If it’s not ready the first time, it’ll be ready the second or third time.