Configure URLs under index.html to point to the corresponding host (Render, Heroku)

[KostaSav]

Example:

1. when https://hp-api.onrender.com/ is used, then the route to All Characters should point to https://hp-api.onrender.com/api/characters
2. when https://hp-api.herokuapp.com/ is used, then the route to All Characters should point to https://hp-api.herokuapp.com/api/characters

The URLs should be made configurable, which means that they will be stored somewhere and based on the host used, the corresponding API basepath is invoked.