katex: make it position: relative to fix broken layout

[fushar]

The Katex's MathML output has the following CSS:

.katex .katex-mathml {
    clip: rect(1px, 1px, 1px, 1px);
    border: 0;
    height: 1px;
    overflow: hidden;
    padding: 0;
    position: absolute;
    width: 1px;
}

The position: absolute rule breaks the layout when a problem is rendered within a course. This is because some course-related divs use something like height: calc(100vh - xxx) to maintain a sticky header. This is incompatible with MathML output above as it somehow sticks at the bottom of the page and causes unnecessary blank space.

With this, the MathML output does not break the layout as the parent div .katex now has position: relative.

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