Add form validation on the client side

I haven't had a chance to implement everything on the server side yet.

This makes it so that you can only submit the form once media has been upload and the required fields are filled in.  Note there is an error causing some of the validation to not work: https://github.com/unl/wdntemplates/issues/816

#123
An input with id "channels" does not exist. Using a fieldset and legend is probably the best way to provide context for all of the channel inputs.