More work on alerts, see rest of commit message:

- Removed base64 encoding for alerts, this added to the size and provided no funtionality.

- Made the alert icon in the header become red (new_alert.png) when you have new alerts. This only happens on template render at the moment, will investigate a jQuery solution.

- Renamed some variables inside users.backend.get_alerts().

- Fixed the alerts unit tests to make them pass after removal of b64encode.
Added verifications to alerts.

- Alerts will not be displayed if the user (by default) does not exist. This has also been overridden in posts to check that the post still exists. There still needs to be a few more unit tests for the alert functionality.

- Fixed an issue with the variable name change in alerts.