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/utils/validate.py

@@ -2,12 +2,12 @@
 from bookwyrm.settings import DOMAIN, USE_HTTPS
 
 
-def validate_url_domain(url, default="/"):
+def validate_url_domain(url):
     """Basic check that the URL starts with the instance domain name"""
     if not url:
-        return default
+        return None
 
-    if url in ("/", default):
+    if url == "/":
         return url
 
     protocol = "https://" if USE_HTTPS else "http://"
@@ -16,4 +16,4 @@ def validate_url_domain(url, default="/"):
     if url.startswith(origin):
         return url
 
-    return default
+    return None