Set sensible defaults as pinned items in groups and projects

What does this MR do and why?

Implements #404883 (closed). See #404883 (comment 1372082739) for more context.

Screenshots or screen recordings

	Before	After
Project
Group

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How to set up and validate locally

1. The default pins are only show for users who haven't edited their pins yet. To reset your user to that state, do this:

   user = User.first # or whatever your user is
   user.update(pinned_nav_items: {}) # "{}" is also the database default value for this JSONB column

2. Enable the new nav:

   1. Enable the feature flag:

      echo "Feature.enable(:super_sidebar_nav)" | rails c

   2. Enable the user setting from the user dropdown:

      

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