Only strip periods from Gmail addresses

@DSASanFrancisco/portal-members

This PR narrows the scope of the period stripping behavior in the normalize_email function to only apply to @gmail.com and @googlemail.com domains. As far as I know, only Google treats their consumer Gmail addresses as "period-agnostic"; periods do affect their G Suite email services (see this link). there might be some custom domain legacy stuff that are also period agnostic but this is not a currently supported Google product.