🎨 🔦 restrict themes storage to local-file-store

refs #6982
- this was handled before by the old ConfigManager
- this logic belongs into the storage logic itself
- for now we only allow uploading themes via the local-file-store

[ci skip]

core/server/storage/index.js

@@ -11,7 +11,15 @@ function getStorage(type) {
     type = type || 'images';
 
     var storageChoice = config.get('storage').active[type],
-        storageConfig = config.get('storage')[storageChoice];
+        storageConfig;
+
+    // CASE: we only allow local-file-storage for themes
+    // @TODO: https://github.com/TryGhost/Ghost/issues/7246
+    if (type === 'themes') {
+        storageChoice = 'local-file-store';
+    }
+
+    storageConfig = config.get('storage')[storageChoice];
 
     // CASE: type does not exist
     if (!storageChoice) {