Deprecate and remove override constants

[hellopablo]

We have the following constants which all serve to avoid sending real people emails on non-production environments:

• EMAIL_OVERRIDE (send all email to this specific address)
• EMAIL_WHITELIST (allow email to these address regexes)
• APP_DEVELOPER_EMAIL (where to send alerts)

We don't really need all of them, but they all serve some sort of purpose.

I think we should keep EMAIL_OVERRIDE and EMAIL_WHITELIST. Where the former is a last minute "send email which is sent to here" and the latter being a filter of whether to attempt to send the email or not.

APP_DEVELOPER_EMAIL is really a legacy error catcher, but this isn't really used any more in favour of error handler drivers.