#166 add event type message template

[aleevas]

For each event type we should be able to configure default message, on path like admin/structure/rng/event_types/manage/node.event/messages/ Added basic stuff, like schema, routes, draft forms, basic callback to get/set default messages. Added functionality that create real Messages for newly created events, based on Default messages from event type config. https://github.com/dpi/rng/issues/166

[freelock]

Merged into freelock/rng.