MDL-40908 core_tag: replaced 'flag' add_to_log call with an event
MDL-40908 core_tag: created a 'tag_unflagged' event
MDL-40908 core_tag: created a 'tag_deleted' event
MDL-40908 core_tag: created a 'tag_created' event
MDL-40908 core_tag: created an 'item_untagged' event