Add GetInitialJukebox to app.proto

[peddybeats]

• rpc GetInitialJukebox(GetInitialJukeboxRequest) -> GetInitialJukeboxResponse

message GetInitialJukeboxRequest { repeated Track seedTrack; }

mesage GetInitialJukeboxResponse { oneof { Error error; Jukebox jukebox; } }

message Track { string id; string name; string album; repeated Artist artists; string image-uri; }

message Artist { string id; string name; }