Acception version and remote url for PUT /api/siteextensions/{id}

E.g. 
```
PUT http://localhost:60741/api/siteextensions/hello
{
  remote: "https://www.myget.org/F/siteextensions-test/"
}
```
Status: `200 OK`
Let's skip the id in the file name. Also, make it plural. So always SiteExtensionSettings.json