This commit prepares the frontend code to be consumed by webpack.
It is a hack: In theory, modules should be declaring and importing the
modules they depend on and the globals they expose directly.
However, that requires significant per-module work, which we don't
really want to block moving our toolchain to webpack on.
So we expose the modules by setting window.varName = varName; as
needed in the js files.
The code here used to live in hotkey.js. Its complicated calling
protocol made it difficult to unit test. We are also trying to
slim down hotkey.js.
Our arrow navigation for things like `#stream_filters` has always
been kind of awkward, since it's difficult to get the focus to
their list items. This commit does nothing to fix that yet.