Add discord button to Thrive articles

[kkartunov]

Design: https://marvelapp.com/prototype/69f39fc/screen/85778895/handoff

[kkartunov]

JIRA: https://topcoder.atlassian.net/browse/PROD-1803

[kkartunov]

Ready for QA on beta. Please check @SathyaJayabal
Should be on any article details page like for instance https://beta-community-app.topcoder.com/thrive/articles/long-short-term-memory-networks-lstm

[SathyaJayabal]

verified on beta