Admin panel login form

Some minor clean up and some logic around admin panel.

I tried to implement the math support with Katex instead of MathJax and see if it is faster to render on the page, but I failed miserably. Your tex code uses so many things that Katex doesn't support, including `&=`, `\\` for forced line break, `\begin{align}`, etc. I gave up after an hour.

Anyway, hope some of these changes help.