Fine tune footer styling

[mmmavis]

Follow-up for PR #2998

To make sure things match what we have on the mockups.

• hover state
• form error state for form related stuff we'll revisit when form standardization work is done.

[kristinashu]

Please have @natalieworth and @sabrinang review the PR when it's ready!

[sabrinang]

@mmmavis for the "Mozilla is a global non-profit ..." text please change this to .body-small.
For dark theme .body-small links and other paragraph links, let's change the links to be in $light-blue.

[kristinashu]

@mmmavis can I add one more update to this ticket (if not I can open a separate one). Please update the two mozilla.org links (logo & "Visit us at mozilla.org") to go to foundation.mozilla.org instead and say "Visit us at foundation.mozilla.org". Abigail from legal says if we're talking about the not for profit then we shouldn't be linking to moz.org.

[mmmavis]

@kristinashu sounds good!