Convert registration to Field component

This converts most fields in the registration form to use the Field component,
except for the phone number, which is a left as a separate task because of the
country dropdown menu.

res/css/structures/auth/_Login.scss

@@ -15,6 +15,7 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
+// TODO: Should be removable once all fields are using Field component
 .mx_Login_field {
     width: 100%;
     box-sizing: border-box;