Make usernames case insensitive

It would be great if Zeppelin would normalizes case of usernames after authentication (let's say, to lower case).

We noticed user sometimes authenticate using different case (for example, john.doe@epsilon.com , next time john.dow@Epsilon.com / John.dow@Epsilon.com etc).

Apparently authentication backends don't case about case (for example, we use LDAP).
It breaks at least one thing - `notebook-authorization.json`'s `owners` field with different cases and users don't see their own notebooks - even though they are in fact are still owners. They can only see a subset of notebooks, that they created when logged in with exact same username.

Would be nice if Zeppelin normalize case, after user successfully authenticates in.