Setup the User's Birthday field to accept input in local format.

#### What's this PR do?
- Configures the User's Birthday field to accept date in the format specific to site language setting

#### How should this be manually tested?
1. Check that user registration works with birthday in US date format, e.g. `05/28/1988`
2. As an admin [change](http://dev.dosomething.org:8888/admin/config/regional/date-time) site's short date format to the EU format, e.g. `18/09/2014 - 00:37`
3. Check that user registration works with birthday in EU date format, e.g. `28/05/1988`
4. Optional: reset the format, install `en-gb` language, switch to it, repeat step 3

#### Any background context you want to provide?
This change utilizes undocumented Date module functionality by changing date input format to hardcoded value `site-wide`. It can be found in `date_elements.inc:618`:

```php
/**
 * Determine the input format for this element.
 */
function date_input_format($element, $field, $instance) {
  if (!empty($instance['widget']['settings']['input_format_custom'])) {
    return $instance['widget']['settings']['input_format_custom'];
  }
  elseif (!empty($instance['widget']['settings']['input_format'])
              && $instance['widget']['settings']['input_format'] != 'site-wide') {
    return $instance['widget']['settings']['input_format'];
  }
  return variable_get('date_format_short', 'm/d/Y - H:i');
}
```

As you can see `site-wide` value makes date field determine its input format from site's short date format setting. This settings is changing automatically when changing site's default language.

> Note: When extra languages installed, it's also possible to [manage](http://dev.dosomething.org:8888/admin/config/regional/date-time/locale) date settings per each language. In this case the date format is determined from the values set on the form that represents site's current language.

#### Warning
At the moment when date field format is set to magic `site-wide` value, but it's not represented on the field's configuration form. If form saved manually, the magic value will be lost.
I will create a ticket on drupal.org and provide a patch to utilize this functionality properly.

##### Progress:
1. [ ] Issue reported to drupal.org
2. [ ] Patch provided
3. [ ] Code included to Date module dev version
4. [ ] Code released in Date module stable version

#### What are the relevant tickets?
Reference: [#DS-335](https://jira.dosomething.org/browse/DS-335)