Create item lend form and supporting JS.

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:

* Polish the LendForm model form. Make the 'item_id', 'user_id', 'due_out',
  'due_back', 'out', and 'back' model fields accessible through the form. Use
  DateInput and DateTimeInput widgets where appropriate, and give the resultant
  HTML elements a type of `date` and `datetime` instead of the default `text`.
* Write JS to make the form nicer to use and more accessible. (see below for
  function names and descriptions) Make functions `clearInput`, `fillDate`, and
  `fillDateTime` callable from buttons within the form, and make
  `checkDateTimeInputs` and `checkDateInputs` fire as soon as the DOM is ready.
* Clean up JS with JSLint. (no warnings emitted!) Add JSLint-style comments into
  code to make future de-linting easier.

JS function prototypes and docstrings:

function clearInput(inputName);
> Clear input `input_name`.

function fillDate(inputName);
> Populate input `input_name` with the current date.

function fillDateTime(inputName);
> Populate input `input_name` with the current date and time.

function checkDateInputs();
> 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.
>
> For the definition of a valid date string, see:
> http://developers.whatwg.org/common-microsyntaxes.html#valid-date-string
>

function checkDateTimeInputs();
> Check to see if the browser supports datetime 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 datetime inputs, find all datetime inputs and fiddle with
> their attributes.
>
> For the definition of a valid datetime string, see:
> http://developers.whatwg.org/common-microsyntaxes.html#local-dates-and-times
Polish /{item,tag,lend}/<id> pages.

Add links to each page leading to the update/edit and delete pages. All links
are contained within `p.controls` tags.

Improve wording of headlines and paragraphs. For example, make header state
"Notes About This Tag" instead of "Notes".

Convert some headers into <p> tags. Also show less information by default. For
example, only show "tags attached to this item" if tags are actually attached to
that item. These two changes make it much easier to find useful information
about an item. Less time is spent wading through useless information and
excessive headers.