Change note dates should use recommended date style

Dates should use the [`govuk_admin_template` standards](https://github.com/alphagov/govuk_admin_template#date-formats), which are based
on the [recommended style for displaying dates](https://www.gov.uk/design-principles/style-guide/style-points#style-dates-and-times).
Show timestamps for the London timezone
Merge pull request #117 from alphagov/timestamp-improvements

Timestamp improvements