Give embedded image files with transparency a checkered background

Summary:
Have a consistent rendering experience between File page and file embedded in a task for PNG image files with an alpha channel:
Show a checkered background on the embedded image instead of a white background; show full contrast when hovering over the embedded image.

Closes T15473

Test Plan:
See steps in T15473 - compare an image file embedded in a task before and after applying this patch.

• test in Task description
• test in Task comment
• test in Task comment inside something weird like a table

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15473

Differential Revision: https://we.phorge.it/D25294