feature: add case insensivity to username and email

Closes #74

As asked by Bernardins and SpaceCoop, the idea is to allow users to enter BenoiT@startinblox.com and be recongnized as benoit@startinblox.com, even though the RFC states that the domain part of email address should be considered case-sensitive, no email provider really do that. The idea is to avoid painful login process when users sadly registers with a capital and then do not get how to log back.

That modification may be an issue because on some instances I can already see people have usernames with capitals in it.

What do you think @calummackervoy or @jbpasquier or someone ?