Store CROMWELLURL not as an env var

[sckott]

So i'm nearly certain now env vars are global across users of the shiny cromwell app, so the URL here needs to be set using a different method.

And while we're at it support more than one server cause folks may have that use case. I don't want to go back to passing a url as a param, but may have to. If that's the route then use a function as the default? e.g,

cromwell_jobs <- function(..., url = find_url()) {
    ...
}

# where find_url is
find_url <- function(url = NULL) {
    # look in an internal package env
}

[sckott]

plus Token handling too