Move get_args out of dataset_ui

[mccalluc]

There is still a call to get_args in dataset_server below, so I'm not sure that it would be much cleaner just to move one and not the other. I do wonder if there is some way the contributions input could get its default value from the server, but the default value isn't really something that is reacting to user input, so I don't see how that would work.

The right way to do this might be more clear to me with more familiarity with Shiny.

Originally posted by @mccalluc in #41 (comment)