feature/fix-collection-nav

This PR fixes the collection nav on the index and "Add Collection" views so that the nav is consistent on both views and only shows the user what they have permission to access. Previously, the "Add Collection" view was showing *all* collections in the navbar, even though the user may not have access to all collections.

@jazahn Can you review this?

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[FLASH-142](https://jira.huit.harvard.edu/browse/FLASH-142)