Fix datepicker in default theme.

- Don't use mm/dd/yyyy format as this will be parsed inconsistently by PHP's
strtotime() depending on locale settings. Use ISO date format instead.

- Make sure the "This Week" quicklink works even if the week crosses into
  the following month.
Merge pull request #1126 from solarissmoke/datepicker-fixes

Datepicker issues in default theme.