PHPCRED-36: Automatically Check if Updated Version is available

As bugs could lead to credentials being exposed, would be wise to ensure that all installs stay up to date.

The easiest way to do so is probably to have a routine that checks a pre-defined URL to see if an updated version is available and raise a notification if a newer version is

Requirements

- Admin should be able to disable the check
- Notification should only appear for Superusers
- Seperate checks should be made for Stable and Dev branches
- A URL to Release Notes (probably on bentasker.co.uk) should be included (including how to apply the update)
- The update should never happen automatically