Fix tag positioning to live within katex-html parent (#1721)

* Fix tag positioning to live within katex-html parent

`.tag` has `position: absolute`, but this positions it relative to the nearest
parent with a non-static `position` (see
https://developer.mozilla.org/en-US/docs/Web/CSS/Containing_block).
This PR makes `.katex-display > .katex > .katex-html` that parent.

* Update screenshots

src/katex.less

@@ -571,6 +571,7 @@
 
         > .katex-html {
             display: block;
+            position: relative;
 
             > .tag {
                 position: absolute;