PLT-2698 Integrations improvements (#2883)

* Fixed Integrations link to show up for non-admins (when enabled) and when only slash commands are enabled

* Updated BackstageSidebar to only show enabled integrations

* Added placeholder to integrations list when none exist

* Added loading spinner to integrations pages when they're loading

webapp/i18n/en.json

@@ -902,8 +902,10 @@
   "get_team_invite_link_modal.helpDisabled": "User creation has been disabled for your team. Please ask your team administrator for details.",
   "get_team_invite_link_modal.title": "Team Invite Link",
   "installed_commands.add": "Add Slash Command",
+  "installed_commands.empty": "No commands found",
   "installed_commands.header": "Slash Commands",
   "installed_incoming_webhooks.add": "Add Incoming Webhook",
+  "installed_incoming_webhooks.empty": "No incoming webhooks found",
   "installed_incoming_webhooks.header": "Incoming Webhooks",
   "installed_integrations.creation": "Created by {creator} on {createAt, date, full}",
   "installed_integrations.delete": "Delete",
@@ -912,6 +914,7 @@
   "installed_integrations.token": "Token: {token}",
   "installed_integrations.url": "URL: {url}",
   "installed_outgoing_webhooks.add": "Add Outgoing Webhook",
+  "installed_outgoing_webhooks.empty": "No outgoing webhooks found",
   "installed_outgoing_webhooks.header": "Outgoing Webhooks",
   "integrations.command.description": "Slash commands send events to external integrations",
   "integrations.command.title": "Slash Command",