Handle pending changes for contributions

This adds the pending tx data to pending contributions (after adding or
vetoing, until the tx is mined). It also disables the veto button while
pending.

app/models/contribution.js

@@ -1,5 +1,5 @@
 import EmberObject, { computed } from '@ember/object';
-import { isEmpty } from '@ember/utils';
+import { isEmpty, isPresent } from '@ember/utils';
 import bignumber from 'kredits-web/utils/cps/bignumber';
 import moment from 'moment';
 
@@ -24,6 +24,8 @@ export default EmberObject.extend({
   time: null,
   ipfsData: '',
 
+  pendingTx: null,
+
   init () {
     this._super(...arguments);
     if (isEmpty(this.details)) this.set('details', {});
@@ -35,6 +37,10 @@ export default EmberObject.extend({
 
   jsDate: computed('iso8601Date', function() {
     return moment(this.iso8601Date).toDate();
+  }),
+
+  hasPendingChanges: computed('pendingTx', function() {
+    return isPresent(this.pendingTx);
   })
 
 });