Image Zoom/Lightbox for Images in Issues and Merge Requests

When screenshots are inline'd into Issues and Merge Requests, it can be hard to see what they're trying to communicate because the important details are too small and there's no easy way to open an image in a larger view, e.g. a "lightbox". I've been having this problem a lot recently.

This would be especially useful on mobile, where things like the following are common:

Alternatively, it would probably work if we just made images link to their source files, which is a much simpler solution.