Lightbox for quick image zoom

Proposal

• When clicking on an image in an MR or issue, first open it in a lightbox
• Have an icon on the image to open the original in new tab if user wants the original image (not shown in concept)
• Similar to how Facebook lets users view photos

Why

• Improve the user experience of quickly clicking on an image to see it in full screen, then simply click away off the image to return to the page.
• Currently opens image in a new tab, which is okay but would be nice to not keep opening new tabs

Concept

Related to #14411 (closed)