Normalized email and domain columns in Email

[Daksh2000]

Fixes: https://github.com/pypi/warehouse/issues/17925

In this PR, I have added 2 columns -> normalized_email and domain (both of which are derived from email itself) So, made 2 helper functions to get the corresponding values of normalized_email and domain

[ I did not use "normalize" property of "email_validator" library because according to it - after using it -> The local part (before @) remains case-sensitive (which I thought was defeating our purpose, so I went with custom function)

Also, Added migrations accordingly, and fixed some test cases.