feat(accordion): add appropriate tabindex on disabled

- Add `tabindex="-1"` to accordion group header when disabled and remove when not to assist with accessibility

Closes #4067
Closes #5990

src/accordion/accordion.js

@@ -1,4 +1,4 @@
-angular.module('ui.bootstrap.accordion', ['ui.bootstrap.collapse'])
+angular.module('ui.bootstrap.accordion', ['ui.bootstrap.collapse', 'ui.bootstrap.tabindex'])
 
 .constant('uibAccordionConfig', {
   closeOthers: true

src/accordion/index.js

@@ -1,4 +1,5 @@
 require('../collapse');
+require('../tabindex');
 require('../../template/accordion/accordion-group.html.js');
 require('../../template/accordion/accordion.html.js');
 require('./accordion');

template/accordion/accordion-group.html

@@ -1,6 +1,6 @@
 <div role="tab" id="{{::headingId}}" aria-selected="{{isOpen}}" class="panel-heading" ng-keypress="toggleOpen($event)">
   <h4 class="panel-title">
-    <a role="button" data-toggle="collapse" href aria-expanded="{{isOpen}}" aria-controls="{{::panelId}}" tabindex="0" class="accordion-toggle" ng-click="toggleOpen()" uib-accordion-transclude="heading"><span uib-accordion-header ng-class="{'text-muted': isDisabled}">{{heading}}</span></a>
+    <a role="button" data-toggle="collapse" href aria-expanded="{{isOpen}}" aria-controls="{{::panelId}}" tabindex="0" class="accordion-toggle" ng-click="toggleOpen()" uib-accordion-transclude="heading" ng-disabled="isDisabled" uib-tabindex-toggle><span uib-accordion-header ng-class="{'text-muted': isDisabled}">{{heading}}</span></a>
   </h4>
 </div>
 <div id="{{::panelId}}" aria-labelledby="{{::headingId}}" aria-hidden="{{!isOpen}}" role="tabpanel" class="panel-collapse collapse" uib-collapse="!isOpen">