Allow API to assign new user to a specific organization (#21775)

* Allow API to assign new user to a specific organization

* Add defer block to test

* Add API tests and return 400 instead of 500 for bad orgId

* Minor test improvements

docs/sources/http_api/admin.md

@@ -224,10 +224,13 @@ Content-Type: application/json
   "name":"User",
   "email":"user@graf.com",
   "login":"user",
-  "password":"userpassword"
+  "password":"userpassword",
+  "OrgId": 1
 }
 ```
 
+Note that `OrgId` is an optional parameter that can be used to assign a new user to a different organization when [auto_assign_org](https://grafana.com/docs/grafana/latest/installation/configuration/#auto-assign-org) is set to `true`.
+
 **Example Response**:
 
 ```http