redirects: Refactor redirect code to use central helper function.

This commit introduces two new functions in 'url_encoding.py' which
centralize two common patterns for constructing redirect URLs. It
also migrates the files using those patterns to use the new
functions.

zerver/lib/url_encoding.py

@@ -97,3 +97,10 @@ def near_pm_message_url(realm: Realm,
     ]
     full_url = '/'.join(parts)
     return full_url
+
+def add_query_to_redirect_url(original_url: str, query: str) -> str:
+    return original_url + "?" + query
+
+def add_query_arg_to_redirect_url(original_url: str, query_arg: str) -> str:
+    assert '?' in original_url
+    return original_url + "&" + query_arg