Add helper to refer views back to http referers safely

In most cases, we want to return back to where we came from after
performing an action. It's not safe to return to an arbitrary referer,
so this streamlines using the util validator to verify the redirect and
fall back on regular redirect params if the referer is outside our
domain.

bookwyrm/tests/test_utils.py

@@ -22,8 +22,8 @@ class TestUtils(TestCase):
 
     def test_invalid_url_domain(self):
         """Check with an invalid URL"""
-        self.assertEqual(
-            validate_url_domain("https://up-to-no-good.tld/bad-actor.exe"), "/"
+        self.assertIsNone(
+            validate_url_domain("https://up-to-no-good.tld/bad-actor.exe")
         )
 
     def test_default_url_domain(self):