Enable math support by default

[adam-p]

Too many people are confused as to why math support doesn't work out of the box for them. I had set it as disabled by default for privacy concern reasons -- you're sending your math to Google for rendering. But... if you don't use the feature, then the privacy concern doesn't affect you. And if you do use (or try to use) the feature, you're unlikely to be dissuaded by the privacy concern (but you are likely to be super confused as to why it doesn't work).

The privacy concern description is still present on the options page.

If we wanted to get fancy (and create a lot of work for ourselves), the first time the user tried to render some math we would put up a box explaining the situation and asking if they wanted to continue.

[adam-p]

Implemented in rev e1f6cba