Add jquery.validate plugin with a basic error.

Unfortunately (or fortunately, depending on how you look at it), this
validation _replaces_ actual HTML5 form validation. At least the HTML5
markup remains the same. Once Safari supports HTML5 form validation, we
can revisit it.

Right now it only applies to docset creation (name is required); do
reuse this method! See #435 #148 #225 #402 #442.

[finishes #58978622]