Added an expand/compress image button on the image view box for Mastodon. 😆

It solved the trouble while there be images with a very long width or height.

Feature:
- Add an expand/compress image button on the image view box
- Scroll the zoomed image with mouse pan-by-grabbing or mouse scrolling

https://github.com/tootsuite/mastodon/commit/6a2db10f767879b9072b6c020ebadac4f985ed34