XSS Preview Issue fix

This PR allows us to use the SafeHtmlValidator in govuk_content_models to protect agains creation of a SpecialistDocument with invalid HTML or which contains Javascript. This means that a SpecialistDocument should not get into the system rather than catching the error later in the chain when we would like to publish a document. 

In order to give the user some feedback regarding the document they are creating we now also give the user an error when previewing the document instead of rendering anything which contains Javascript.

We have also managed to remove some noise inside the SpecialistDocument class around errors allowing the SpecialistDocument to not require awareness of errors and validations. This is now pushed into the validators instead.