Bump govuk_content_models to 7.2.0

This gives us the extended list of Whitehall kinds for an artefact,
and extra artefact factories for tests.
Split ResultSetPresenter#present into multiple bits

Split the base pagination logic in ResultSetPresenter into a private
method. This will make it easier to subclass this presenter and inherit
some of the core pagination behaviour whilst changing how the results
are presented.
Allow tagged artefacts to be grouped by format type

Add a new presenter, GroupedResultSetPresenter, which will group the
items in the provided result set by their kind/format.

Requests to /with_tag.json?group_by=format will receive a different
response built by this presenter, instead of the regular
ResultSetPresenter.

For now, the list of groupings is hard-coded in the new presenter, along
with the labels which should be returned for each group.
Sort the grouped results in the order they are defined

Each group must appear in a predetermined order, so that the most
important content (like services) appear above other formats.

This adds a sort operation on the grouped results to make sure they
are arranged in the same order as the hash in which they are defined.
Reject values for "group_by" that aren't "format"

For now, we only group by format, so explicitly reject requests for
another type of grouping.
Exclude content which doesn't fit into any format group

This might be something we change in the future, but right now we
shouldn't display any content which is a format type not specified
for any group.
Make the grouped_results method clearer
Merge pull request #160 from alphagov/specialist-sector-content-types

Allow content for a tag to be grouped by format