Rewording: Integrations become (store) plugins

We're keeping the `store-integrations-list` extension point name for backwards compatibility, otherwise I renamed all "integration" occurences in the views and also the routes.

Note: We still refer to external software like WooCommerce, Drupal, Magento and such as "integrations", I think that distinction makes sense.

Closes #3594.

BTCPayServer/Models/StoreViewModels/PluginsViewModel.cs

@@ -0,0 +1,6 @@
+namespace BTCPayServer.Models.StoreViewModels
+{
+    public class PluginsViewModel
+    {
+    }
+}