Deleting 'self' user remains logged in

[uhppoted]

If the admin user deletes their own user they remain logged in and until the cookie expires can log back in.

Rework so that:

• [x] Deleting self logs out automatically
• [x] Deleting any user clears the cookies for that user
• [x] Cookie UIDs are checked against the current users list
• [x] Check what happens if the same user is logged in on e.g. two different browsers

[twystd]

• Added checks for user is deleted to all authentication paths
• Cleared session cookies for users that are no longer valid
• A deleted user is automatically logged on on page refresh/timeout
• Posted a security notice in the README
• Notified any known users who might possibly be affected

Closing as (provisionally) completed.