Find out the source of all countries with geocodes

### Question to answer in this research

What trusted source of geo data we could use in the project? Data we need to store are
- country name
- country geocode (e.g. US/UK/FR)
- postal code format
- list of postal codes (since could require frequent updates, it's **optional**)

### Research reason

The research is needed because we are going to handle worldwide postal codes, not the US only.

### Ideas to start with

- [ ] Django have a great plugin https://github.com/yourlabs/django-cities-light . Probably there is something similar for flask as well.

Start with the required country name, geocode, postal-code format, and optional postal-code list in the issue body. Evaluate django-cities-light and comparable Flask-compatible sources for worldwide coverage, trustworthiness, and update frequency. Done means documenting a recommended source and how its data should be maintained.

Written by the indexing model from the issue text.