PLT-6554 Add config setting to control enabling API version 3 (#6835)

* Add config setting to control enabling API version 3

* Update help text for APIv3 config setting (#6843)

* Update configuration_settings.jsx

* Update en.json

config/config.json

@@ -21,6 +21,7 @@
         "EnableOnlyAdminIntegrations": true,
         "EnablePostUsernameOverride": false,
         "EnablePostIconOverride": false,
+        "EnableAPIv3": true,
         "EnableLinkPreviews": false,
         "EnableTesting": false,
         "EnableDeveloper": false,