Add option for notifications to only be sent if the user is not on the private message page

Users currently receive (email) notifications of every single private message. There should be an option in the settings to only send notifications if/when the user is not currently on the private message page.

Methodology:

• Every time a user accesses the page, their last access time should be updated.
• Every time the AJAX on the page checks for new messages, their last access time should be updated
• *A setting will be created on the configuration page, asking if users should receive notifications for all messages, or only when they are away from the screen
• If the setting is to only send notifications when the user is not on the page, a setting will be provided to let admins set the number of seconds after which a user should be considered away.
• If the setting is to only send notifications when the user is not on the page, then when it comes time to send a notification, the user's last notification time will be checked. if it has been under the number of seconds set on the configuration page, no notification will be sent.
• Users will have the option to configure these settings for their own account in their account settings edit page.