* Fix subforem caching leak in top navigation

The main sidebar fragment cache was not including subforem_id in its cache
key, causing navigation content (logo, signup links, community name) to be
cached and served across different subforems incorrectly.

Changes:
- Updated fragment cache key in layouts/application.html.erb to include
  RequestStore.store[:subforem_id]
- Updated community_name helper to use subforem-aware memoization to
  prevent cross-subforem pollution within a request

This ensures each subforem has its own cached navigation content.

* Top bar caching

* Fixie