Add achievementsPath to NamespaceType for graphql

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What does this MR do and why?

Add achievementsPath to NamespaceType for graphql as described in Expose achievementsPath against NamespaceType (#458269 - closed).

The graphql query for namespace needs an achievementsPath so the user can be redirected to the namespace's achievements.

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. In rails console enable the experiment fully

   Feature.enable(:achievements)

2. From GraphQL Explorer - /-/graphql-explorer
3. Send the following query:

{
  currentUser {
    userAchievements {
      nodes {
        achievement {
          namespace {
            achievementsPath
          }
        }
      }
    }
    namespace {
      achievementsPath
    }
  }
}

The achievementsPath should be populated for group namespace, and null for user namespace.