Persisted user settings and notable bugfixes in Portable Text Editor and migration CLI tooling

User settings (like desk list sort orders, view modes, and global search history) are now stored securely server-side by Sanity. This means that these settings do not need to be re-selected across devices or browsers, and will be persisted wherever a user logs into their Studio.

• Fixes a bug in the Portable Text Editor where merging two text blocks could result in data loss in the topmost block when several editors are inside the document.
• The schema migration CLI tooling now always places its output in the project root, and provides better messaging about this.