Fix issue with `session` object

[davidruvolo51]

(Backdating this issue)

When using the handlers, it is required that session is passed into the function. For example,

js$addCSS("#myelem", "my-class", session = session)

This is a bit inconvenient and it's easy to forget this argument. More work is needed to determine how to use session without having to pass it as an argument.

[davidruvolo51]

Fixed.

In the server, the file must be loaded as:

source("path/to/js_handlers.R", local = TRUE)