Invalidate NAV cache on different page count

When removing a page the NAV cache must be invalidated. Only using the
LastEdited field is not enough, so a count() of the Page has been
appended the cache key.
Put a link to forms/Form.ss in Includes/Form.ss

This make the theme compatible with SilverStripe 3.1. Without this
commit it is used the default Form.ss template provided by sapphire.
Merge branch 'dev'