feat(timepicker): add placeholder for seconds input

- Adds `SS` placeholder for seconds input

Closes #5257

template/timepicker/timepicker.html

@@ -18,7 +18,7 @@
       </td>
       <td ng-show="showSeconds" class="uib-separator">:</td>
       <td class="form-group uib-time seconds" ng-class="{'has-error': invalidSeconds}" ng-show="showSeconds">
-        <input style="width:50px;" type="text" ng-model="seconds" ng-change="updateSeconds()" class="form-control text-center" ng-readonly="readonlyInput" maxlength="2" tabindex="{{::tabindex}}" ng-disabled="noIncrementSeconds()" ng-blur="blur()">
+        <input style="width:50px;" type="text" placeholder="SS" ng-model="seconds" ng-change="updateSeconds()" class="form-control text-center" ng-readonly="readonlyInput" maxlength="2" tabindex="{{::tabindex}}" ng-disabled="noIncrementSeconds()" ng-blur="blur()">
       </td>
       <td ng-show="showMeridian" class="uib-time am-pm"><button type="button" ng-class="{disabled: noToggleMeridian()}" class="btn btn-default text-center" ng-click="toggleMeridian()" ng-disabled="noToggleMeridian()" tabindex="{{::tabindex}}">{{meridian}}</button></td>
     </tr>