Add task label model, CRUD operations and assign/unassign label feature

[JakubJW]

We need to implement task labels management system. Labels should be managed per board, as every board may have different labels. As for other fields within entity model, i think colour and name should be enough.

[OresFCK]

Done -> https://github.com/JakubJW/trello-clone-backend/pull/6