[Cashtab] Address content jumping for Send button

Summary: Related to T2332. When the Primary and Disabled buttons swap, content jumped as the disabled button had a smaller amount of padding. This diff makes the padding between both buttons equal.

Test Plan:
cd web/cashtab && npm start
Navigate to send screen
Input a single character into the XEC Address input to trigger the disabled button
Observe that the content on the screen does not jump.

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D11447