Switch to API version 8 (gateway and REST) (#1405)
* Deprecate PrivateChannelCreateEvent * Deprecate PrivateChannelDeleteEvent * Use JSON body for ban requests * Add handling for schema errors * Ignore private messages for self user if channel not in cache * Implement new presence cache * Add private channel LRU handling * Add missing message types * Throw exception for old constructors * Add parsing error handling in ErrorResponseException * Add CacheFlag#ONLINE and disable it in createDefault * Add CacheFlag#getPrivileged (commit: 0ab5896)