Added target_guid and related functionalities to river.
Deprecates add_to_river in favor of elgg_create_river_item.
added tests for elgg_create_river_item()
Added target_guid to database schema.
Prefetches target entity for river.
Replaced add_to_river() with elgg_create_river_item().
Adds better validation for river parameters.
Uses _elgg_retrieve_cached_entity() instead of retrieve_cached_entity().
Merge pull request #5394 from juho-jaakkola/target_guid

Adds target guid to river