Enable creators to choose which countries donations are allowed to come from

[Changaco]

Closes #1102 (even though this implementation is actually based on territoriality, not nationality).

[codecov]

Codecov Report

Attention: Patch coverage is 59.09091% with 9 lines in your changes are missing coverage. Please review.

Project coverage is 81.19%. Comparing base (dfb6764) to head (3008f25).
Report is 2 commits behind head on master.

❗ Current head 3008f25 differs from pull request most recent head 27fec00. Consider uploading reports for the commit 27fec00 to get more accurate results

Additional details and impacted files

Files	Coverage Δ
liberapay/i18n/base.py	92.15% <100.00%> (+0.22%)	⬆️
liberapay/payin/common.py	90.88% <50.00%> (-0.49%)	⬇️
liberapay/models/participant.py	87.79% <66.66%> (+0.43%)	⬆️
liberapay/exceptions.py	78.66% <50.00%> (-0.32%)	⬇️

... and 26 files with indirect coverage changes

[Changaco]