feat(accordion): use attribute for heading insertion

- Change to use `uibAccordionHeader` attribute for insertion point for header transclusion

Closes #5324
Closes #5396

BREAKING CHANGE: This changes to use the `uibAccordionHeader` attribute instead of a `<span>` element for inserting the custom header HTML. If you use a custom template for the accordion group, please add this attribute to the appropriate location.

src/accordion/accordion.js

@@ -123,8 +123,9 @@ angular.module('ui.bootstrap.accordion', ['ui.bootstrap.collapse'])
     link: function(scope, element, attrs, controller) {
       scope.$watch(function() { return controller[attrs.uibAccordionTransclude]; }, function(heading) {
         if (heading) {
-          element.find('span').html('');
-          element.find('span').append(heading);
+          var elem = angular.element(element[0].querySelector('[uib-accordion-header]'));
+          elem.html('');
+          elem.append(heading);
         }
       });
     }

src/accordion/docs/readme.md

@@ -44,6 +44,8 @@ The body of each accordion group is transcluded into the body of the collapsible
 
 Instead of the `heading` attribute on the `uib-accordion-group`, you can use an `uib-accordion-heading` element inside a group that will be used as the group's header.
 
+If you're using a custom template for the `uib-accordion-group`, you'll need to have an element for the heading to be transcluded into using `uib-accordion-header` (e.g. `<div uib-accordion-header></div>`).
+
 ### Known issues
 
 To use clickable elements within the accordion, you have override the accordion-group template to use div elements instead of anchor elements, and add `cursor: pointer` in your CSS. This is due to browsers interpreting anchor elements as the target of any click event, which triggers routing when certain elements such as buttons are nested inside the anchor element.

template/accordion/accordion-group.html

@@ -1,7 +1,7 @@
 <div class="panel" ng-class="panelClass || 'panel-default'">
   <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 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"><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">