3605: Adopt some more refactoring from upstream r=heinrich5991 a=Kaffeine
- Backport 'Make spectators stay specs on map changed'
- Adopt upstream refactoring: Mark several functions as 'const'
- CGameContext::SendVoteStatus: Hide the trick for 64 clients
- GameController: Add OnPlayerConnect() like in the upstream
- Move DDRace-specific player initialization code to the Controller. Dependencies:
- CGameControllerDDRace: Add Score() getter
- Get rid of gamemode.h with confusing/conflicting definitions (re-defined `GAME_NAME`)
- Replace remaining int64_t by int64 for the sake of consistency
I checked the changes in-game (I ran a server and tested with two players)
## Checklist
- [x] Tested the change ingame
- [ ] Provided screenshots if it is a visual change
- [ ] Tested in combination with possibly related configuration options
- [ ] Written a unit test if it works standalone, system.c especially
- [ ] Considered possible null pointers and out of bounds array indexing
- [ ] Changed no physics that affect existing maps
- [ ] Tested the change with [ASan+UBSan or valgrind's memcheck](https://github.com/ddnet/ddnet/#using-addresssanitizer--undefinedbehavioursanitizer-or-valgrinds-memcheck) (optional)
Co-authored-by: Alexander Akulich <akulichalexander@gmail.com>