Allowing more options to style pretix shops (#1585)

* Fix caching issues in SASS compilation

* Allow to set a custom page background color

* Allow to disable round corners

* Support larger header pictures

* Allow to show title despite header

* Move language picker

* FIx widget styles

src/pretix/presale/templates/pretixpresale/base.html

@@ -51,7 +51,7 @@
 {{ html_page_header|safe }}
 {% block above %}
 {% endblock %}
-<div class="container">
+<div class="container main-box">
     {% block page %}
     {% endblock %}
     <footer>