Simplify how we handle redacted events

include/mtx/events_impl.hpp

lib/structs/events/aliases.cpp

lib/structs/events/collections.cpp

lib/structs/events/guest_access.cpp

lib/structs/events/history_visibility.cpp

lib/structs/events/join_rules.cpp

lib/structs/events/pinned_events.cpp

lib/structs/responses/common.cpp

tests/events.cpp

tests/responses.cpp