Simplify return type of most route handlers

src/client_server/account.rs

src/client_server/alias.rs

src/client_server/backup.rs

src/client_server/capabilities.rs

src/client_server/config.rs

src/client_server/context.rs

src/client_server/device.rs

src/client_server/directory.rs

src/client_server/filter.rs

src/client_server/keys.rs

src/client_server/media.rs

src/client_server/membership.rs

src/client_server/message.rs

src/client_server/presence.rs

src/client_server/profile.rs

src/client_server/push.rs

src/client_server/read_marker.rs

src/client_server/redact.rs

src/client_server/report.rs

src/client_server/room.rs