Image Previewer should display embedded ICO files correctly (BB-13560)

Right now when viewing a Pull Request in BitBucket, .ico files are not displayed correctly in many cases, depending on the way the file was saved. .ico files can be saved in a couple of different formats. If the .ico file only contains a single image, then it already is displayed correctly. But this is unusual. Typically, .ico files will have multiple images embedded inside of them. I use a free program called IcoFX to generate my *.ico files, and I know it stores the following different versions of the same file inside the single file:

1. 512x512 - 32 colors
2. 256x256 - 32 colors
3. 48x48 - 32 colors
4. 32x32 - 32 colors
5. 16x16 - 32 colors
6. 48x48 - 8 colors
7. 32x32 - 8 colors
8. 16x16 - 8 colors
9. 32x32 - 4 colors
10. 16x16 - 4 colors

With files like these, currently the BitBucket image previewer shows a blank square instead of the image.

What it should do: display the largest available image with the most colors available. So in the example above it would show the 512x512 (32 colors) version.