Polish model forms and lend-id-update-form.html

Make model forms more explicit about when a widget is being referenced. Make
'due_out' and 'due_back' widgets in model form `LendForm` date inputs instead of
regular text inputs.

Add a "reset" button to lend-id-update-form.html. Change text on the
JS-triggering buttons from "Populate" to "Today", so as to more clearly imply
what the triggered JS will do.
Add javascript to ensure support of date inputs

Add a JS function named `checkDateInputs`. From the docstring:

> Check to see if the browser supports date inputs.
>
> Create a dummy element that will never be attached to the DOM tree and use it
> to probe the capabilities of the browser. If the browser does not provide a
> custom control for date inputs, find all date inputs and fiddle with their
> attributes.

Also clean up JavaScript with JSLint.