delete invoices (#1652)

* include all meta fields as template variables
* support invoice preview via command
* support deletion of generated invoices

templates/invoice/renderer/text.txt.twig

@@ -1,19 +1,16 @@
 {% for key, value in model.toArray %}
-{{ key }}:
+{{ key }}
 {% if value is not empty %}
-{{ value|multiline_indent('    ') }}
+{{ value|multiline_indent('	') }}
 {% endif %}
-
 {% endfor %}
 {% for entry in model.calculator.entries %}
 ---
-
 {% set items = model.itemToArray(entry) %}
 {% for key, value in items %}
-{{ key }}:
+{{ key }}
 {% if value is not empty %}
-{{ value|multiline_indent('    ') }}
+{{ value|multiline_indent('	') }}
 {% endif %}
-
 {% endfor %}
 {% endfor %}