Support optional MathJAX rendering

[aarchiba]

As #20 but for MathJAX. This permits users to see simple mathematical expressions in rendered form. This is probably simpler than #20 in that it is much less likely to trigger unintentionally; some folks have MathJAX running over all their email. Minimalist implementation would probably just mean plonking a reference to MathJAX in the page header HTML, as MathJAX is already designed to leave non-LaTeX unmodified.

[aarchiba]

An example of MathJAX in action: https://arxiv.org/abs/1905.08679

The submitted abstract will have been plain text containing, for example:
[...] a hardening of the slope at about $10^{18.7}$ eV [...]