Replace links to Gitter with link to Slack workspace

CONTRIBUTING.md

@@ -12,7 +12,7 @@ The following communication channels are available:
 
  * [openvsx issues](https://github.com/eclipse/openvsx/issues) - for general issues (bug reports, feature requests, etc.)
  * [open-vsx.org issues](https://github.com/EclipseFdn/open-vsx.org/issues) - for issues related to [open-vsx.org](https://open-vsx.org/) (outage reports, requests about extensions and namespaces, etc.)
- * [Gitter chat](https://gitter.im/eclipse/openvsx) - for questions and project-related discussion
+ * [Slack workspace](https://join.slack.com/t/openvsxworkinggroup/shared_invite/zt-2y07y1ggy-ct3IfJljjGI6xWUQ9llv6A) - for questions and project-related discussion
  * [Developer mailing list](https://accounts.eclipse.org/mailing-list/openvsx-dev) - for organizational issues (e.g. elections of new committers)
 
 ## How to Contribute

README.md

@@ -2,7 +2,7 @@
 
 [![Gitpod Ready-to-Code](https://img.shields.io/badge/Gitpod-ready--to--code-blue?logo=gitpod)](https://gitpod.io/#https://github.com/eclipse/openvsx)
 [![Contribute](https://www.eclipse.org/che/contribute.svg)](https://workspaces.openshift.com#https://github.com/eclipse/openvsx)
-[![Join the chat at https://gitter.im/eclipse/openvsx](https://badges.gitter.im/eclipse/openvsx.svg)](https://gitter.im/eclipse/openvsx)
+[![Slack workspace](https://img.shields.io/badge/Slack-Join%20workspace-4A154B?logo=slack&logoColor=white)](https://join.slack.com/t/openvsxworkinggroup/shared_invite/zt-2y07y1ggy-ct3IfJljjGI6xWUQ9llv6A)
 [![CI](https://github.com/eclipse/openvsx/workflows/CI/badge.svg)](https://github.com/eclipse/openvsx/actions?query=workflow%3ACI)
 
 Open VSX is a [vendor-neutral](https://projects.eclipse.org/projects/ecd.openvsx) open-source alternative to the [Visual Studio Marketplace](https://marketplace.visualstudio.com/vscode). It provides a server application that manages [VS Code extensions](https://code.visualstudio.com/api) in a database, a web application similar to the VS Code Marketplace, and a command-line tool for publishing extensions similar to [vsce](https://code.visualstudio.com/api/working-with-extensions/publishing-extension#vsce).

webui/src/default/menu-content.tsx

@@ -162,10 +162,10 @@ export const MobileMenuContent: FunctionComponent = () => {
             </Link>
         </MobileMenuItem>
         <MobileMenuItem>
-            <Link href='https://gitter.im/eclipse/openvsx'>
+            <Link href='https://join.slack.com/t/openvsxworkinggroup/shared_invite/zt-2y07y1ggy-ct3IfJljjGI6xWUQ9llv6A'>
                 <MobileMenuItemText>
                     <ForumIcon sx={itemIcon} />
-                    Community Chat
+                    Slack Workspace
                 </MobileMenuItemText>
             </Link>
         </MobileMenuItem>
@@ -205,8 +205,8 @@ export const DefaultMenuContent: FunctionComponent = () => {
         <MenuLink href='https://github.com/eclipse/openvsx/wiki'>
             Documentation
         </MenuLink>
-        <MenuLink href='https://gitter.im/eclipse/openvsx'>
-            Community
+        <MenuLink href='https://join.slack.com/t/openvsxworkinggroup/shared_invite/zt-2y07y1ggy-ct3IfJljjGI6xWUQ9llv6A'>
+            Slack Workspace
         </MenuLink>
         <MenuRouteLink to='/about'>
             About