API: Add createdAt field to /api/users/:id (#19475)

It would help for auditing purposes to have the user's created date exposed via the API along with the other fields already exposed via the API.

pkg/api/user_test.go

@@ -35,6 +35,7 @@ func TestUserApiEndpoint(t *testing.T) {
 					IsDisabled:     false,
 					IsExternal:     false,
 					UpdatedAt:      fakeNow,
+					CreatedAt:      fakeNow,
 				}
 				return nil
 			})
@@ -63,7 +64,8 @@ func TestUserApiEndpoint(t *testing.T) {
 				"authLabels": [
 					"LDAP"
 				],
-				"updatedAt": "2019-02-11T17:30:40Z"
+				"updatedAt": "2019-02-11T17:30:40Z",
+				"createdAt": "2019-02-11T17:30:40Z"
 			}
 			`