fix(accordion): fix unexpected routing

- Change anchor tag to `div` to prevent anchor tag from triggering route change

BREAKING CHANGE: This causes the cursor style to be removed from the heading - implement CSS on the `accordion-toggle` class accordingly

Closes #4792

src/accordion/docs/demo.html

@@ -1,3 +1,9 @@
+<style>
+  .accordion-toggle {
+    cursor: pointer;
+  }
+</style>
+
 <div ng-controller="AccordionDemoCtrl">
   <script type="text/ng-template" id="group-template.html">
     <div class="panel {{panelClass || 'panel-default'}}">

src/accordion/test/accordion.spec.js

@@ -151,7 +151,7 @@ describe('uib-accordion', function() {
     var scope, $compile;
     var element, groups;
     var findGroupLink = function(index) {
-      return groups.eq(index).find('a').eq(0);
+      return groups.eq(index).find('.accordion-toggle').eq(0);
     };
     var findGroupBody = function(index) {
       return groups.eq(index).find('.panel-collapse').eq(0);

template/accordion/accordion-group.html

@@ -1,7 +1,7 @@
 <div class="panel {{panelClass || 'panel-default'}}">
   <div class="panel-heading" ng-keypress="toggleOpen($event)">
     <h4 class="panel-title">
-      <a href tabindex="0" class="accordion-toggle" ng-click="toggleOpen()" uib-accordion-transclude="heading"><span ng-class="{'text-muted': isDisabled}">{{heading}}</span></a>
+      <div tabindex="0" class="accordion-toggle" ng-click="toggleOpen()" uib-accordion-transclude="heading"><span ng-class="{'text-muted': isDisabled}">{{heading}}</span></div>
     </h4>
   </div>
   <div class="panel-collapse collapse" uib-collapse="!isOpen">