Fixed cache.populate() leaving behind old settings

refs https://github.com/TryGhost/Team/issues/1070

- if the cache is repopulated, keys that already existed in the cache but that don't exist in the new group of settings were left in the cache
- added a `.clear()` method that removes all items from the cache and call it when populating so the cache only contains what it was last populated with
  - deletes properties on the internal content object so that references aren't lost

ghost/custom-theme-settings-service/lib/cache.js

@@ -12,8 +12,16 @@ module.exports = class CustomThemeSettingsCache {
     }
 
     populate(settings) {
+        this.clear();
+
         settings.forEach((setting) => {
             this.content[setting.key] = setting.value;
         });
     }
+
+    clear() {
+        for (const key in this.content) {
+            delete this.content[key];
+        }
+    }
 };