Notifications: Invalidate cache when updating a notification

[r-a-y]

If you're on your notifications page and you click on the "Read" action link for a notification, the user's notifications cache isn't invalidated.

Problem is due to not pulling up the user ID before updating the notification.

I've attached a patch with unit tests.

The patch adds a new hook - 'bp_notification_before_update' - which mirrors the other before_save and before_delete hooks and moves the cache clearing to bp-notifications-cache.php.

[johnjamesjacoby]

In 9887:

Notifications: Improve all_for_user cache deletion.

• Introduce bp_notification_before_update action which fires before a Notification is updated
• Introduce bp_notifications_clear_all_for_user_cache() as wrapper for wp_cache_delete() with appropriate cache group and key
• Introduce bp_notifications_clear_all_for_user_cache_before_update() hooked to bp_notification_before_update action, to handle cache deletion before a notification is updated

This change fixes a bug where marking single notifications as read/unread would not show anticipated results, as the all_for_user cache was not being properly cleared.

Props r-a-y. Fixes #6445.

[johnjamesjacoby]

In 9888:

Notifications: Tests for all_for_user cache deletion.

Props r-a-y. See #6445. See r9887.