Payment History receipt filenames for transactions with same date get unique suffix (#4605)

[willy-b]

• Submitted a ticket for my issue if one did not already exist: Fixes Payment History modal receipt filenames should handle multiple same-day transaction edge case #4605
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed): Squashed to two commits, the "feature" and the tests, rebased onto tag v0.12.4dev-RC1

Test Plan:

1. Enable Brave Payments, create wallet, fund it (use testnet if you want to save BTC)
2. Trigger reconciliation multiple times by editing //ledger-state.json reconcileStamp to be in past (e.g. set to 100)
3. Open Brave Payments -> Payment History
4. Observe "Receipt Link" column
5. Receipt filenames should all be unique
6. All Receipt filenames after the 1st for a given day should end in "${n}.csv", like "Brave_Payments${MM-D(D)-YYYY}_${n}.csv" (where n=2 for the 2nd transaction on that day, n=3 for 3rd
7. Unit tests verifying this should be present and passing

To check unit tests, run:

NODE_ENV=test mocha --compilers js:babel-register --recursive test/unit/lib/ledgerExportUtilTest.js

and check the block of tests beginning w/ addExportFilenamePrefixToTransactions

[diracdeltas]

++ thanks!