Make Google account email customizable

[GDay]

When a new Google account is created, an email is being send out to the new hire with the details for that. This message is not customizable at the moment. It's a plain text email message, which can be seen here: https://github.com/chiefonboarding/ChiefOnboarding/blob/master/back/integrations/emails.py#L10

This should be customizable, while still keeping the email and password fields. This could go directly under settings, I don't think this will have to be customizable for individual new hires.