Control backend API response schema by protobuf

[OldPanda]

Now the response is defined twice at

• https://github.com/OldPanda/green-tiles/blob/master/backend/services/github_types.go
• https://github.com/OldPanda/green-tiles/blob/master/src/types.ts

Introducing protobuf could help remove the possible conflicts in the future as the project evolves, and it'll also benefits parsing performance on responses.