Shift responsibilities between Manual and PublishingAPIManual

Manual now only validates that the incoming manual conforms to the JSON schema.
PublishingAPIManual is responsible for:

* publishing to the Publishing API
* transforming the incoming payload
* sanitisation
Add specs for invalid or empty manual payloads
Move ValidationError to separate file
Shift responsibilities between Section and PublishingAPISection

Section now only validates that the incoming section conforms to the JSON schema.
PublishingAPISection is responsible for:

* publishing to the Publishing API
* transforming the incoming payload
* sanitisation
Add specs for invalid or empty section payloads
Merge pull request #46 from alphagov/untangle-incoming-and-publishing-api-models

Untangle incoming and publishing api models