Fix inconsistent return status code on creating resources

The return status codes, content-negotiation and prefer-header is not consistent to the general REST patterns, the other openEHR endpoints/resource handling and neither to general principle described in Overview spec page.

It is stated that a return empty-body should returned with status 201, where as that is normally a 204. This is controlled with `Prefer: return=minimal` request header.

ref: https://specifications.openehr.org/releases/ITS-REST/Release-1.0.1/definitions.html#definitions-adl-2-template-post

PS: it looks like this is not only for template, but the inconsistency is present to few other places.