Clean up formatting for bitcoin wallet backup

[jkup]

• Submitted a ticket for my issue if one did not already exist.
• 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).

Fix #4807

Auditors @bsclifton @bbondy

Test Plan

1. Open Brave
2. Go to Preferences > Payments
3. Click Advanced Settings
4. Try both Printing and Saving your backup keys. They should look like the screenshots below.

[bsclifton]

Tested out the change, looks good! This might be a good one for a unit test (if you wanted to do that as a follow up; totally optional 😄 ). The formatting logic could be broken out into ledgerUtil. This would let us easily test different inputs (null, undefined, not string, etc) to ensure none of them cause a problem

Merging! 😄