Update Discord Nav/Footer Link

[mmcbride2929]

TL/DR

This PR updates the discord link within the Nav & Footer to navigate the user to the FCC discord invite page

Overview of Change

links.js

• update discord href to navigate to discord invite screen

Header.test.js

• imported screen & render from testing-library
• wrote test to make sure we are rendering 'Discord' as the third item

Footer.test.js

• imported screen & render from testing-library
• wrote test to make sure we are rendering 'Discord' as the third item

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[readysetagile]

comments within.

• use the correct link
• remove the "third link" test
• remove the changed files which did not require changes

[jduffey]

All comments were addressed. Discord link now points to John's invite link. Approving.