Sanity Studio v6.8.0: Structure Tool list virtualization fix and other bugfixes

This release prevents Structure Tool list items from being erroneously hidden after expanding a collapsed pane, improves CLI authentication handling, fixes release view rate-limit issues, and resolves WebSocket connection errors.

🐛 Notable bugfixes and improvements

• Fixes a bug in which Structure Tool list items would erroneously be hidden after expanding a collapsed list pane.
• Fixes occasional rate-limit errors when opening large published or archived Content Releases in Studio by spacing history API requests the same way active releases already do.
• Fixes a memory leak in Studio's RxJS↔React bridging by upgrading react-rx to 4.2.5.
• Fixes an issue causing WebSocket is already in CLOSING or CLOSED state. error to occur after downloading an asset.