Allow overriding the edit form action

This makes it the same as the add form. Useful if you are doing
something weird using Google's blobstore.create_upload_url(..) API.

src/wagtailmedia/templates/wagtailmedia/media/edit.html

@@ -30,7 +30,7 @@
         <div class="row row-flush nice-padding">
 
             <div class="col10 divider-after">
-                <form action="{% url 'wagtailmedia:edit' media.pk %}" method="POST" enctype="multipart/form-data" novalidate>
+                <form action="{% block action %}{% url 'wagtailmedia:edit' media.pk %}{% endblock %}" method="POST" enctype="multipart/form-data" novalidate>
                     {% csrf_token %}
                     {% if next %}<input type="hidden" value="{{ next }}" name="next">{% endif %}
                     <ul class="fields">

tests/templates/wagtailmedia/media/edit.html

@@ -0,0 +1,3 @@
+{% extends "wagtailmedia/media/edit.html" %}
+
+{% block action %}/somewhere/else/edit{% endblock %}

tests/test_views.py

@@ -468,6 +468,37 @@ def test_extends(self):
         self.assertContains(response, "sweet-form-row")
         self.assertContains(response, "sweet-stats")
 
+    def test_action_block(self):
+        with self.settings(
+            TEMPLATES=[
+                {
+                    "BACKEND": "django.template.backends.django.DjangoTemplates",
+                    "DIRS": [os.path.join(os.path.dirname(__file__), "templates")],
+                    "APP_DIRS": True,
+                    "OPTIONS": {
+                        "context_processors": [
+                            "django.template.context_processors.debug",
+                            "django.template.context_processors.request",
+                            "django.contrib.auth.context_processors.auth",
+                            "django.contrib.messages.context_processors.messages",
+                            "django.template.context_processors.request",
+                            "wagtail.contrib.settings.context_processors.settings",
+                        ],
+                        "debug": True,
+                    },
+                }
+            ]
+        ):
+            response = self.client.get(
+                reverse("wagtailmedia:edit", args=(self.media.id,))
+            )
+            self.assertEqual(response.status_code, 200)
+            self.assertTemplateUsed(response, "wagtailmedia/media/edit.html")
+            self.assertContains(
+                response,
+                '<form action="/somewhere/else/edit" method="POST" enctype="multipart/form-data" novalidate>',
+            )
+
     def test_post(self):
         # Build a fake file
         fake_file = ContentFile("A boring example song", name="song.mp3")