Allow sharing and saving images

[quinncomendant]

On the iOS app, there is no option to share an image in an article (not sure about Android; does it allow this?).

Ideally, images have several functions available:

• Share… (invoke the share sheet (UIActivityViewController))
• Save to Photos
• Copy
• Copy Subject
• Show Text
• Look Up

These functions would be available when long-pressing an image inline in an article, or by tapping a share icon 📤 in the image viewer.