Operator Rendering Rework

[stefnotch]

I should use an approach like React for rendering the MathML, so that I can efficiently update it.

To render an operator without the hacks, I can insert a

<mrow class="nya"><mo stretchy="true" symmetric="true" rspace="0" lspace="0" minsize="0">|</mo></mrow>

with the following CSS

.nya {
  width: 0px;
  height: 0px;
  margin-left:-2px;
  margin-right: 2px;
  box-shadow: 0 -1px 0 #000;
  margin-top:-4px;
  margin-bottom:-4px;
}

I have to make sure to only ever draw that where an mrow is (easy) and I have to makes sure to add the form="prefix/infix/postfix" attribute to each mo. Otherwise the automatic rendering would treat these two differently

<mrow><mo>+</mo><mn>∞</mn></mrow>

<mrow> <mrow class="nya><mo>|</mo></mrow>  <mo>+</mo><mn>∞</mn></mrow>