Added embedded activities to the work package resource

Signed-off-by: Alex Coles <alex@alexbcoles.com>
Created activity model
Created representer for activities
Fixed paths to modules
Allowed API to render nil for activity's user
Specified host for the test env
Fixed wrong path in application_helper_test.rb
Minor indentation fix in WorkPackageRepresenter

[ci skip]

Signed-off-by: Alex Coles <alex@alexbcoles.com>
Merge pull request #1534 from opf/feature/api_work_package_embedded_activities2

Feature/api work package embedded activities