Add a simple test mode (#1181)

- [x] Provide data model and configuration toggle
- [x] Allow to delete individual test orders
- [x] Add tests
- [x] Add a prominent warning message to the backend if test mode orders exist (even though test mode is off), as this leads to wrong statistics
- [x] Decide if and how to generate invoices for test orders as invoice numbers cannot be repeated or should not have gaps.
- [x] Decide if and how we expose test orders through the API, since our difference pull mechanism relies on the fact that orders cannot be deleted.
- [x] Decide if and how we want to couple test modes of payment providers?
- [ ] pretix.eu: Ignore test orders for billing
- [ ] Adjust payment providers: Mollie, bitpay, cash, fakepayment, sepadebit

![download](https://user-images.githubusercontent.com/64280/53009081-fe420d80-343a-11e9-8361-b8511c988598.png)

src/pretix/static/pretixbase/scss/_theme.scss

@@ -1,6 +1,14 @@
 .sidebar-nav li > a > .fa {
   color: $navbar-inverse-bg;
 }
+.nav .testmode a {
+  background: $brand-warning;
+  color: black;
+  font-weight: bold;
+  &:hover, &:active, &:focus {
+    background: $btn-warning-border;
+  }
+}
 
 .btn {
   border-width: 0px;

src/pretix/static/pretixcontrol/scss/main.scss

@@ -604,3 +604,7 @@ details summary {
     color: $brand-danger;
     text-decoration: line-through !important;
 }
+
+h1 .label {
+    display: inline-block;
+}

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

@@ -281,6 +281,12 @@ details summary {
 form.download-btn-form {
     display: inline;
 }
+.alert-testmode {
+    margin-top: 20px;
+}
+h2 .label {
+    display: inline-block;
+}
 
 @import "_iframe.scss";
 @import "_a11y.scss";