Remove webpack devtool eval

Summary:
webpack devtool eval wraps everything in an eval that doesn't
wrap files in closures, resulting in leaking `var`s to the
global namespace. While I hear the eval method is faster,
I'm a bit scared of changing semantics for that. (And we
can work on speeding this up after, maybe with a plugin
to closurify everything before using devtool eval.)

Test Plan:
 #yolo
tested in KAQuest repo, where devtool eval was causing globals to
be emitted for vars.

Auditors: alpert