chore: Move from Slack to Discord (#572)

Author: Betree

.github/ISSUE_TEMPLATE/config.yml

@@ -3,12 +3,12 @@ contact_links:
   - name: Issue Tracker
     url: https://github.com/opencollective/opencollective/issues
     about: Repository to track issues across all our projects
-  - name: Slack Channel
-    url: http://slack.opencollective.com/
+  - name: Discord Channel
+    url: https://discord.opencollective.com
     about: Engage with the Open Collective community
   - name: Documentation
     url: https://docs.opencollective.com/help/
     about: Documentation for Open Collective
   - name: Security Policy
     url: https://github.com/opencollective/opencollective/security/policy
-    about: Please report security vulnerabilities as outlined here
+    about: Please report security vulnerabilities as outlined here

README.md

@@ -98,6 +98,5 @@ URL: https://rest.opencollective.com/
 
 ## Discussion
 
-If you have any questions, ping us on Slack
-(https://slack.opencollective.com) or on Twitter
+If you have any questions, ping us on [Discord](https://discord.opencollective.com) or Twitter
 ([@opencollect](https://twitter.com/opencollect)).