Get rid of 'using json = nlohmann::json'

CMakeLists.txt

io.github.NhekoReborn.Nheko.yaml

src/Cache.cpp

src/Cache_p.h

src/ChatPage.cpp

src/encryption/DeviceVerificationFlow.cpp

src/encryption/Olm.cpp

src/encryption/SelfVerificationStatus.cpp

src/timeline/EventStore.cpp

src/timeline/TimelineModel.cpp