Add method to localise dates for DST support

Dates are local to Europe/London now (since integrating
datainsight_recorder v0.4.1) and as such, local datetime should be used
when querying as well.
Fix DateSeriesPresenter validations for DST

The daylight saving time support from the datainsight_recorder 0.4.1 can
result in weeks an hour longer or shorter than 7 days. A better test is
to verify that the start and end datetimes are at midnight according to
their UTC offset.
Merge branch 'master' into bump-recorder-gem
Merge pull request #11 from alphagov/bump-recorder-gem

Fix querying and tests for DST support