Webtrends reports

This does not handle the special case of keymetrics.

By default it collects daily. This means that for a given date range it
will make a query for every day in that range in an inclusive fashion
(e.g. the last day will be 'end_at' to 'end_at + 1' day). This is
because the api aggregates date ranges by default. Most of the logic is
doing this querying and parsing the date formats used by webtrends and
the nested json returned by the api.

The main integration type test is test_collect_parse_and_push.

This also includes some changes to data_parser to handle empty special
fields (though this is not needed in this collector) and consistent
formatting of datetimes when idMapping is specified.

The use of this new collector will be documented fully in the collector config
repo.

**Do not merge** until https://github.com/alphagov/performanceplatform-collector/pull/94 and https://github.com/alphagov/performanceplatform-collector/pull/93 have been merged:

Also waiting on a bug fix. Will update when this is in.