Convert registration phone number to Field component

Now that we have prefix support in the Field component, we can also convert the
phone number with country dropdown on registration.

res/css/structures/auth/_Login.scss

@@ -95,6 +95,7 @@ limitations under the License.
     flex: 1 1 auto;
 }
 
+// TODO-START: Remove up to TODO-END when all the country dropdowns are in Fields
 .mx_Login_field_prefix {
     height: 38px;
     padding: 0px 5px;
@@ -141,3 +142,4 @@ limitations under the License.
     margin: 3px;
     vertical-align: top;
 }
+// TODO-END: Remove from TODO-START when all the country dropdowns are in Fields