upgrade to katex 0.12

[williamstein]

... or instead switch to mathjax -- see #4734

NOTE: @haraldschilly upgraded us to katex 0.12 in #4740, but his and my testing of that wasn't good, since all the math is typeset wrong (e.g., no italics, missing display mode, etc.). so I reverted that:

I have no idea what that is broken, but I would rather invest time in switching to only mathjax than trying to debug this myself.

[williamstein]

In case it isn't clear, the font of math is not in italics, but should be. This is very serious since with latex the semantic meaning of italics is very different than roman font.