63 files that no longer have a character, and what that means for you

Summary:
I removed the reliance on global `_` from several files in perseus.

We still have a global `_` since pretty much all of graphie needs
it :(. But perseus files should use `var _ = require` now.

This isn't super well tested, or enforced (since ke uses global `_`),
but i think i caught a lot of cases.

Test Plan:
I stubbed out `window._` with a proxy that yelled at me for every
global underscore property access. Then I made some functions to
ignore these errors inside of some khan-exercises things, and I
poked around a bit.

I very much doubt I fixed all the usages

Auditors: alex