Added margin-bottom to the transfer funds button on about:preferences#payments

[luixxiul]

Labelled QA/no qa needed since this will be redone with #6047. Please skip QA for this.

• 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 #6048

Auditors: @bbondy

Test Plan:

1. Open https://jsfiddle.net/LnwtLckc/5/
2. Click register
3. Make sure "Transfer BTC" button has the margin-bottom on add funds dialog on about:preferences#payments:

[bsclifton]

Previous comment deleted- I didn't read your instructions, @luixxiul 😆

After opening the jsfiddle link and adding handler, it shows as expected and margin looks great 😄 Nice cleanup!

[luixxiul]

Labelled QA/no qa needed as this was superseded with a8e5b89 of #6047