fix(signup/serializer): Add validate_email to do case insenstive lookup at signup

[gagantrivedi]

Add validate_email that does a case insensitive lookup
using the email to see a user exists with that email, and returns
the email with all lower case letters

closes #415

[dabeeeenster]

Is the title correct for this PR? "remove case insensitive search on user email" - not sure what removing get_by_natural_key actually achieves here? I feel like we should not allow BEN@flagsmith.com and ben@flagsmith.com to both register, even if it breaks the RFC. I feel like if BEN@flagsmith.com exists in our DB, we should not allow ben@flagsmith.com to register.

Also I don't believe we use the username field currently?

[matthewelwell]

Yeah, this is not the right approach here. We need to ensure that emails are being stored case insensitive on sign up to avoid any issues with this.