Fix #277 -- Embeddable shop (#622)

* Vendor vue.js

* Refactor item_group_by_category to support vouchers

* Widget: Show product list

* Widget: free prices

* Widget: pictures and loading indicator

* Widget: First iframe steps

* Widget: Do not rerender iframe

* Widget: Error handling

* Improve widget

* Widget: localization tech

* Fix invoice style

* Voucher attribute and waiting list

* Add some iframe chrome

* First step to namespaced carts

* More isolation steps

* More cart isolation things

* More cart isolation things

* Mobile stuff

* Show cart on checkout pages

* PayPal and Stripe support

* Enable downloads

* Locale handling

* change text "save URL to this exact page"

* Widget: voucher redemption

* Widget: CSS

* CSS: Responsive

* Widget: CSS improvements

* Widget: Add embedding code generator

* Widget: Error messages and SSL check

* First tests

* Widget: tests

* Don't use IDs in widgets

* Widget: static files caching

src/pretix/static/pretixpresale/scss/main.scss

@@ -201,6 +201,12 @@ body.loading .container {
     text-decoration: none;
 }
 
-.panel-title a[data-toggle="collapse"]:hover {
+.panel-default .panel-title a[data-toggle="collapse"]:hover {
     background-color: #eeeeee;
 }
+
+.panel-primary .panel-title a[data-toggle="collapse"]:hover {
+    background-color: darken($btn-primary-bg, 10%);
+}
+
+@import "_iframe.scss";