Remove disallowed properties from the list of required properties

When generating the frontend schema, also remove disallowed properties
from the list of required properties which are output in the frontend
schema.
Validate input schemas when combining
Correction of schema syntax

The 'string' type is case sensitive

When defining the null type, specify the string `"null"` rather than the
json literal `null`
Regenerated after correction of errors in publisher schema

also removed required properties which are not allowed in frontend
schema
Explicitly set additionalProperties in fixtures
Inject available_translations into links hash

.. when generating frontend schema.

Also explicitly test the scenario where the publisher schema may not
have had any links. In that scenario the frontend schema also needs to
insert some schema boilerplate for the links.
Frontend links definition should define array

Each frontend link is an array of items, so define this as a plural.
Specify that a frontend links definition gets injected
validation script
definition of `frontend_links` should be array
Makefile to validate all frontend examples

this relies on tweaking the validation script to guess which schema to use.