Allow LDAP logins (for on-prem installs) to use email or username 1 1

[jen_at_floyd 0]

This is a fairly minor pain point but on our on-prem Ftrack instance we have LDAP logins enabled and since the login screen placeholder says "Username or Email" we've had a few users get confused when they try to use their email address to log in. We have our "Login attribute" in the LDAP settings set to `sAMAccountName` and I could set it to `mail` instead but then we'd have the same problem but in reverse. It would be nice to maybe have an 'alt login attribute' setting or maybe even have the 'login attribute' be a comma-separated list of values.

As an alternative, a simpler solution might be to allow for on-prem installs to add a bit of explanatory help text to this login window or change the placeholder text for the input box.