v1.21.5

[BannerGen]
1.Prevented sending request to BannerGen when cache exists
2.Improved avatar caching mechanism
[Minor changes]
1.User activity: Changed user_{userid} to member_{userid}
2.Rate limit: Added X-RateLimit-Reset and X-RateLimit-Reset-After header for non-429 responses
3.User: Added PATCH /user/name to update username to latest Discord user name / server nickname
4.Event: Added first_event_after request param for better ordering
5.Discord Integration: Added error notification when fails to update roles
6.Discord Integration: Added config.use_server_nickname, if true, then use nickname in Discord server
7.Configuration: Added config validator to use default value for missing fields