Link authorities overview to authority pages

@maxfliri
@gtrogers
Merge pull request #20 from alphagov/volume_by_authority

Link authorities overview to authority pages
update collections and fakeapi to conform with new group_by/period API response format
move Group model to extensions, add generic Timeseries collection
refactor licensing collections
update graph components to work with new group/period query response format
Merge branch 'adopt-group_by/period-format' into applications-table