Clarify minting of URIs of registries

[nichtich]

Current implementation in dev provides registries endpoints with :id but expects full URIs.

• annotations and mappings have URIs minted by the servers, so they have endpoints with /:id
• concordances have endpoint with /:id but their URIs are not minted by the server (but must start with uriBase)
• concepts and schemes have external URIs

Related #237.

Solution: Allow to configure URIs

a) external URIs => no endpoints with /:id
b) baseURI (set to true for baseUrl + type like no done for annotations and mappings) and freely chosen local id
c) UUID (implies baseURI)

{
  "mappings": {
    "create": {
      "baseURI": true ,
      "idSource": "uuid",
      "idPattern": "..."
    }
}