Removing 'parent' department from NMD description

non-ministerial departments are currently described as being "of" their parent department. This isn't the case as they report independently to ministers. This removes the implied parent/child relationship.
Refactor organisation_display_name_and_parental_relationship

Add a test case covering non-ministerial organisations.
Re-write description building for readability.
Add a test case covering full stops.
Merge pull request #1302 from alphagov/remove_parent_dept_from_NMD_description

Removing 'parent' department from NMD description

https://www.pivotaltracker.com/s/projects/1008986/stories/66542606