api: Add is_remote_server().

This function abstracts the logic to ascertain the source of API
auth request.

zerver/lib/utils.py

@@ -223,3 +223,12 @@ def split_by(array, group_size, filler):
     """
     args = [iter(array)] * group_size
     return list(map(list, zip_longest(*args, fillvalue=filler)))
+
+def is_remote_server(identifier):
+    # type: (Text) -> bool
+    """
+    This function can be used to identify the source of API auth
+    request. We can have two types of sources, Remote Zulip Servers
+    and UserProfiles.
+    """
+    return "@" not in identifier