Make contest with timer visible before joinable

[Eason0729]

As the title said.

And this will also be part of basis backend.

proto draft:

message PublishContestRequest{
 require int32 contest_id = 1;
 // if set, user can only join after this timestamp(admin can bypass restriction
 optional google.protobuf.Timestamp begin = 4;
}

service Contest {
 rpc List(ListContestRequest) returns (ListContestResponse);
 rpc FullInfo(Id) returns (ContestFullInfo);

 rpc Create(CreateContestRequest) returns (Id);
 rpc Update(UpdateContestRequest) returns (google.protobuf.Empty);
 rpc Remove(RemoveRequest) returns (google.protobuf.Empty);

 rpc Publish(PublishContestRequest) returns (google.protobuf.Empty);
 rpc Unpublish(PublishRequest) returns (google.protobuf.Empty);

 rpc Join(JoinContestRequest) returns (google.protobuf.Empty);
}

message UpdateContestRequest {
 message Info {
 optional string title = 1;
 optional string content = 4;
 optional string tags = 6;
 // if there is password: enter original password to disable password, null
 // to keep password. if there isn't: enter new password to set password,
 // null to keep it password disable
 optional string password = 5;
 optional google.protobuf.Timestamp end_of_submission = 7;
 }
 required Info info = 1;
 required int32 id = 2;
 // can prevent duplicate request.
 // It will return cache result if server the request with the same
 // `request_id` has be processed.
 optional string request_id = 3;
}

[Eason0729]

#90