moving m2m field from tags to eventlogs so we can re-use tags with superevent logs

gracedb/events/api/views.py

gracedb/events/migrations/0014_eventlog_tags.py

gracedb/events/migrations/0015_migrate_tag_data.py

...events/migrations/0016_delete_old_tag_event_logs_table.py

gracedb/events/models.py

gracedb/events/permission_utils.py

gracedb/events/view_utils.py

gracedb/events/views.py