Fix highlighting of current user in contributor list

Fixes #72

Introduces a helper to determine if a contributor is the current user.

app/helpers/is-current-user.js

@@ -0,0 +1,16 @@
+import Helper from '@ember/component/helper';
+import { inject as service } from '@ember/service';
+import { alias } from '@ember/object/computed';
+import { isPresent } from '@ember/utils';
+
+export default Helper.extend({
+
+  kredits: service(),
+  currentUser: alias('kredits.currentUser'),
+
+  compute([contributor]) {
+    return isPresent(contributor) && isPresent(this.currentUser) &&
+            contributor.account === this.currentUser.account;
+  }
+
+});