Add support for optional updateDate

[martinstabe]

Standard FT stories display PublishDate unless the UpdateTimestamp flag is set to true.

Default starter kit behaviour is currently the reverse: displaying the latest build date unless a publication date is fixed.

This should be possible to fix by:

1. Require existing publishedDate to be a static date (default could be the scaffolding date)

2. adding two new fields to the config: An UpdateTimestamp Boolean and an optional date UpdateDate which defaults to the current time.

3. adding a condition to views/includes/article-head.html that show "Updated" before the date
   and uses the UpdateDate if UpdateTimestamp is true.

[joannaskao]

Do you have an example showing how FT.com displays "updated" on updated articles? When we spoke about this about a year ago, I was told that since the site doesn't do this, we shouldn't either.