lightbox: Add "share" button in bottom app bar

[chrisbobbe]

The share_plus plugin has an API for sharing a file: https://pub.dev/documentation/share_plus/latest/share_plus/Share/shareXFiles.html

We should try it out, for a button to share the image.

As with #42, I didn't include this in the prototype because I think it'll involve an API request; see there for details.

[gyoussef55]

I would like to work on it