Add App Directory link to Integrations pages (#6731)

* Update installed_commands.jsx

* Update installed_incoming_webhooks.jsx

* Update installed_oauth_apps.jsx

* Update installed_outgoing_webhooks.jsx

* Update integrations.jsx

* Update quick_switch_modal.jsx

* Update en.json

* Decrease help text size in Integrations page

* Fix syntax error

* Tweak help text size based on Asaad's feedback

* Fix indentation

* Update help text classname to backstage-list__help

* Update _backstage.scss

* Remove the horizontal line in integrations page

* Renamed template strings for integrations help links

webapp/components/quick_switch_modal/quick_switch_modal.jsx

@@ -272,7 +272,12 @@ export default class QuickSwitchModal extends React.PureComponent {
 
         let help;
         if (Utils.isMobile()) {
-            help = null;
+            help = (
+                <FormattedMessage
+                    id='quick_switch_modal.help_mobile'
+                    defaultMessage='Type to find a channel.'
+                />
+            );
         } else if (this.props.showTeamSwitcher) {
             help = (
                 <FormattedMessage