lowercase username before replacing unsupported characters

peertube doesn't support uppercase characters or any special characters other than _ in usernames. To handle that, the auth-openid-connect plugin replaces all unsupported characters with _. unfortunately this means that any usernames with uppercase characters in the openId system don't have very good names in peertube:

For example: DancingHippo becomes _ancing_ippo.

To address this, before replacing all unsupported characters, we lowercase the username.