Time machine mode [Z#23129725] (#3961)

Allows organizers to test their shop as if it were a different date and time.

Implemented using a time_machine_now() function which is used instead of regular now(), which can overlay the real date time with a value from a ContextVar, assigned from a session value in EventMiddleware.

For more information, see doc/development/implementation/timemachine.rst

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Co-authored-by: Richard Schreiber <schreiber@rami.io>
Co-authored-by: Raphael Michel <michel@rami.io>

src/pretix/static/pretixcontrol/js/send_form.js

@@ -0,0 +1 @@
+document.forms[0].submit();

src/pretix/static/pretixpresale/scss/_event.scss

@@ -194,7 +194,7 @@ div.front-page {
     padding: 5px;
     text-align: center;
 
-    a {
+    a, .btn-link {
         text-decoration: underline;
         color: white;
     }

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

@@ -39,6 +39,7 @@ $body-bg: #f5f5f5 !default;
 h1 a, .btn {
     text-decoration: none;
 }
+a .fa:first-child { margin-right: 0.5ch }
 /*
 a, .btn-link {
     text-decoration: underline;