Refactor blobs_controller blob and commit methods

What does this MR do and why?

• remove redirect logic from #blob and #commit methods
• create before_action method ensure_blob! and ensure_commit! to handle redirects
• strong_memoizes blob and commit methods
• abstract redirection logic for blobs
• removes if else statement from blob method
• removes calls to @blob in favor of blob
• removes calls to @commit in favor of commit

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