Refactor get_countries_and_studies to get_all_studies; add docstring
Add docstrings
Fix APIs to get all studies and studies by ISO and study_id
Merge branch 'ged-add-api-to-list-countries-and-subnational-regions' of https://github.com/gem/oq-platform into ged-add-api-to-list-countries-and-subnational-regions
Merge branch 'ged-add-api-to-list-countries-and-subnational-regions' into exposure-export-UI-changes