Update plugins to support standalone components

- ThemedComponent wrappers should always import their base component. This ensures that it's always enough to only import the wrapper when we use it.
- This implies that all themeable components must be standalone

→ added rules to enforce this
→ updated usage rule to improve declaration/import handling

.eslintrc.json

@@ -245,6 +245,7 @@
         "rxjs/no-nested-subscribe": "off",     // todo: go over _all_ cases
 
         // Custom DSpace Angular rules
+        "dspace-angular-ts/themed-component-classes": "error",
         "dspace-angular-ts/themed-component-selectors": "error",
         "dspace-angular-ts/themed-component-usages": "error"
       }