Refactoring dialog on about:preferences#payments (l10n)

[luixxiul]

• 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).

Closes #6007

border-bottom on the bottom should not exist (the button is not displayed if Coinbase is enabled)

Auditors: @bbondy @bradleyrichter

Test Plan:

1. Disable Coinbase widget
2. Make sure every button on the dialog on about:preferences#payments works

[luixxiul]

Closing in favor of #6009

the commit was cherry-picked