Allow users to select more entities and events

[felubra]

Events should not be hardcoded.

Requirements:

• Configuration form with module options, and, in this options one to allow using all entities apart from comments, node and user entities
• Function to generate a table with all entities and events (the events can be hardcoded to create, update and delete)
• Removal of VALID_EVENTS constant in OutgoingWebhook.php