Move some procesess to GitHost subclasses

[maciekbanas]

This issue somehow reverts to previous GitStats architecture, where GitHub and GitLab classes existed as subclasses of GitHost. This architecture seemed complex, as it introduced an addittional layer to GitStats logic.
Nonetheless, it had one big advantage: managing on git platform level which engine to choose when process of getting data is divided between two engines, e.g.:

• pulling commits response via GraphQL, but organizing commits authors logins and names via REST.
• pulling repositories via GraphQL, but repos contributors via REST.

Summing up, although structure of classes grows, the process of pulling data will be constructed through readable "bricks".

[maciekbanas]

Some examples to move:

• setting up engines

[maciekbanas]

Developed in #383