First attempt at making items acutally lendable.

It is currently possible to reserve an item for a user. However, it is not
possible to actually lend out and return an item. Additionally, the backend
logic for this functionality is missing -- certain fields are required, and
that's it.

This commit does the following:

* Update the lend editing form. Django's built-in widgets for this purpose are
  not sufficient, and the form will need to be constructed by hand. The new form
  works well, but it's a proof of concept. Only two fields are present, much of
  the form layout should be pushed down into the widget layer, and so on.
* Add JS for auto-populating <input type='date'> fields. The JS functions should
  be renamed, and versions that can manipulate both 'date' and 'datetime' fields
  should be present.
* Expand the underlying form logic. Formerly, the 'out' and 'back' fields would
  be discarded by the form. They are now handled nicely.