Use doseq=True when re-encoding query params in set_query_params

Otherwise, the list of values will be stringified and then URL encoded, resulting in something like locale=%5B%27en%27%5D

From parse_qs docs: Use the urllib.parse.urlencode() function (with the doseq parameter set to True) to convert such dictionaries into query strings.

wagtail/admin/utils.py

@@ -68,5 +68,5 @@ def set_query_params(url: str, params: dict):
     querydict = parse_qs(query)
     querydict.update(params)
     querydict = {key: value for key, value in querydict.items() if value is not None}
-    query = urlencode(querydict)
+    query = urlencode(querydict, doseq=True)
     return urlunsplit((scheme, netloc, path, query, fragment))