[Cashtab] Make disabled send button more explicit

Summary: Related to task T2321. Added light gray color to theme.buttons.secondary.disabledSendBg and replaced the backgroundcolor of SecondaryButton in PrimaryButton.js. SecondaryButton on hover now shows cursor as the universal 'no' sign.

Test Plan:
cd web/cashtab && npm start
navigate to send tab
enter a value into the address or amount field which would cause the Send button to be deiabled
observe the light grey background color
hover cursor over the disabled button
observe the cursor switch to the universal no sign

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D11235