Add system console switch for enabling link previews (#5663)

webapp/i18n/en.json

@@ -1,4 +1,8 @@
 {
+  "admin.customization.enableLinkPreviewsDesc": "Enable users to display a preview of website content below the message, if available. When true, website previews can be enabled from Account Settings > Advanced > Preview pre-release features.",
+  "admin.customization.enableLinkPreviewsTitle": "Enable Link Previews:",
+  "admin.customization.linkPreviews": "Link Previews",
+  "admin.sidebar.linkPreviews": "Link Previews",
   "multiselect.go": "Go",
   "multiselect.instructions": "Use up/down arrows to navigate and enter to select",
   "multiselect.placeholder": "Search and add members",