[Date picker] Support IDs containing dot(s)

Ex: <input type="date" id="start.date" />
Merge pull request #2854 from berubs/v3.0

[Date picker] Support IDs containing dot(s)
Forms: Add input-placeholder mixin

Backported from Compass vNext (0.13), but modified for the 0.12
definition of legacy Firefox.

Fixes gh-2855
Merge pull request #2865 from nschonni/add-placeholder-mixin

Forms: Add input-placeholder mixin
GC Web Usability Intranet theme: Added missing image (fixes #2859).
Merge pull request #2869 from pjackson28/missingintranetimage

GC Web Usability Intranet theme: Added missing image (fixes #2859).
Now hiding .datepicker-format elements when the datepicker polyfill is not loaded.
Merge pull request #2872 from pjackson28/datepicker-format-hide

Now hiding .datepicker-format elements when the datepicker polyfill is not loaded.
Datepicker polyfill: Fixed typo in extends to correct issue in #1170.
Merge pull request #2878 from pjackson28/fixdatepickerextends

Datepicker polyfill: Fixed typo in extends to correct issue in #1170.
Datepicker/Form validation: Updated date modified on working examples pages.