Fix #308 -- Preview for email templates (#438)

* add ajax and dummy api response

* add preview <p> blocks

* finalise order_placed setup

- use <pre> for mail preview
- create dummy data in backend

* fix i18n text conversion

* create fragment template for mail preview

* support i18n in mail preview view

* apply mail fragment in all mail settings

fix mistake in input[lang=en] flag style
add dummy data for all placeholders
apply fragment template to all fields
add exclude option to fragment template

* remove migration file

* add translation mapping & fix field label

remove hardcoded field label
add transblock for translation file

* add test for mail setting preview

* fix code style in preview class

* bug fix in mail preview view

- fixed localised date values
- added locale index mapping
- added tests on multi-language event
- enhanced dummy data

src/pretix/control/templates/pretixcontrol/base.html

@@ -32,6 +32,7 @@
 		    <script type="text/javascript" src="{% static "pretixcontrol/js/ui/main.js" %}"></script>
             <script type="text/javascript" src="{% static "pretixcontrol/js/ui/quota.js" %}"></script>
             <script type="text/javascript" src="{% static "pretixcontrol/js/ui/question.js" %}"></script>
+            <script type="text/javascript" src="{% static "pretixcontrol/js/ui/mail.js" %}"></script>
             <script type="text/javascript" src="{% static "pretixbase/js/asynctask.js" %}"></script>
             <script type="text/javascript" src="{% static "pretixbase/js/asyncdownload.js" %}"></script>
 		{% endcompress %}