Coerce emails to lower

[signebedi]

Like usernames, we should coerce all emails to lowercase.

Originally posted by @signebedi in #239 (comment)

[signebedi]

The point is, emails are not case sensitive, so we should coerce them into lower to prevent situations where the same email is reused with a certain degree of capitalization changes. We use the ilike operator from sqlalchemy, which in theory should handle this at the point of user creation, but I think that it is still worth implementing.

[signebedi]

This change, when pushed, will likely invalidate several API keys, which store user emails as their foreign key...

[signebedi]

Could this maybe have been done more efficiently using a validator in the pydantic model? SOmething like:

@validator('username', 'email')
def to_lower(cls, value):
     return value.lower()