Discord replaces Twitch in Footer

Created by: st0nebraker

This closes #5566 (closed), and /community's #18

Changelog

• Places Discord icon button where the Twitch was in the Footer
• Changes the Community Footer item name to Discord

Open question

• @jdorfman , do we want community's slug to change to => /discord too? Or just the footer link to the community page?

Test

1. Ensure prettier has standardized the proposed changes.
2. Ensure the Discord icon renders & links to the correct page

Staging link:

https://deploy-preview-5570--sourcegraph.netlify.app/