Add JSON schemas for data to be sent to the API

We haven't yet decided exactly how optional fields should be handled;
we want consistent data in the content-store for fields which aren't
present, and some ways of achieving that may involve adding restrictions
for those fields in the schema, but we think that the structure of the
fields themselves is unlikely to change significantly.

This also includes validation tests of minimal and maximal JSON examples,
which are easier to read than the schemas themselves.
Validate JSON request examples against the schemas

These examples were converted by hand from the scraped JSON in
manuals-frontend, and as the schemas evolve they are very likely to
become invalid JSON and inconsistent with the schemas. Validating
the examples against the schemas ensures that this doesn't happen.

This commit also removes some invalid trailing commas and fixes the
manual slug in EIM25525.
Link to examples and schema from README
Add $schema to JSON Schemas

It is recommended the JSON Schemas declare that they are JSON schemas, as
opposed to simply some JSON.

I used a specific pointer to the current version, rather than the "latest" to
document which version we wrote it against.
Disallow unspecified properties in JSON

It makes the process of checking for unsanitary markup easier if we disallow
unspecified keys: we are then free to use the schema to find all string values
to check.

Additionally, it prevents users from defining new fields in the API. This could
potentially be a disadvantage.

The tests needed to be updated so that they still supplied valid data.