Generate 'Enlarge' image in CSS rather than HTML (Linker)

Generate 'Enlarge' image in CSS rather than HTML (Linker)

There is an ungodly number of requests for magnify-clip.png, embedding
it in CSS is probably good for performance.

It also makes it a lot easier to change the icon, or provide a vector
version (SVG or icon font).

We need to be careful to keep new CSS compatible with old HTML (HTML
is cached for up to 30 days in WMF environment), but luckily in this
case this requires just one CSS rule. Follow-up to remove it: I7a920105.

Bug: 69673
Change-Id: I849a2222fdf708573e54fa0658dbef4d9f2f665f