Update unpublishing schema to reflect reality

Neither the explanation or alternative URL are required on an unpublishing in
whitehall.
fix typo
Frontend generator rework.

Build up schema by copying elements of original rather than cloning and
modifying.
Avoid modifying the input
Required links should be preserved in frontend schema
Pass frontend links definition from file
Merge pull request #9 from alphagov/frontend-schema-generator

Frontend schema generator
Add coming_soon details schema and example.
Merge pull request #10 from alphagov/coming_soon

Add coming_soon details schema and example.
Add README for coming soon format
Spike of a dummy server for example content