Speed up the Payment Instruments page (and fraud reviews)

[Changaco]

Store the brand, last four digits, hash, owner name and expiration date of a payment instrument in Liberapay's database.

[codecov]

Codecov Report

Attention: Patch coverage is 76.92308% with 12 lines in your changes missing coverage. Please review.

Project coverage is 80.76%. Comparing base (6557c97) to head (cac531b).
Report is 52 commits behind head on master.

Files with missing lines	Patch %	Lines
liberapay/models/exchange_route.py	76.47%	12 Missing ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
liberapay/constants.py	100.00% <ø> (ø)
liberapay/payin/stripe.py	73.43% <ø> (-0.41%)	⬇️
liberapay/testing/__init__.py	93.16% <100.00%> (+2.51%)	⬆️
liberapay/models/exchange_route.py	67.11% <76.47%> (-0.60%)	⬇️

... and 4 files with indirect coverage changes