feat(pager): toggle tabindex when disabled

- Add `tabindex="-1"` when disabled and remove it when not for accessibility

Closes #5996

Author: wesleycho

src/pager/index.js

@@ -1,4 +1,5 @@
 require('../paging');
+require('../tabindex');
 require('../../template/pager/pager.html.js');
 require('./pager');
 

template/pager/pager.html

@@ -1,2 +1,2 @@
-<li ng-class="{disabled: noPrevious()||ngDisabled, previous: align}"><a href ng-click="selectPage(page - 1, $event)">{{::getText('previous')}}</a></li>
-<li ng-class="{disabled: noNext()||ngDisabled, next: align}"><a href ng-click="selectPage(page + 1, $event)">{{::getText('next')}}</a></li>
+<li ng-class="{disabled: noPrevious()||ngDisabled, previous: align}"><a href ng-click="selectPage(page - 1, $event)" ng-disabled="noPrevious()||ngDisabled" uib-tabindex-toggle>{{::getText('previous')}}</a></li>
+<li ng-class="{disabled: noNext()||ngDisabled, next: align}"><a href ng-click="selectPage(page + 1, $event)" ng-disabled="noNext()||ngDisabled" uib-tabindex-toggle>{{::getText('next')}}</a></li>