Set the base currency for the user through a menu button

The bot will translate the user change request into a `POST` with
parameters
```
{
    chat_id: 123,
    base_currency : ‘USD’
}
```

# Conflicts:
#	commands/settings.js

- Added exchange and coins keyboards
- Refactoring of existing logic to fit the new need of parameters and
pagination

# Conflicts:
#	telegram.js

Features:
- no more going back at each save
- current state shows a • near the label
- send selected coins is ready, waiting for the integration test with
backend
- improvement of the update logic