Implement support for allocating uncommitted changes to lanes

[krlvi]

• Initial type definition of hunk assignment to a lane
• API for listing hunk assignments
• API for creating a new assignment
• Handling of file system changes
• Handling of hunk dependencies (locks)
• Persistence
• Handling of "unapply branch" containing assigned hunks
• Performance: Use a dirty bit signal from the file system watcher to make the "list assignments" endpoint not reconcile when it's not needed
• Add "locked" to the assignment type (useful to the UI)
• Report to the UI when the assign_hunk operation rejects an operation due locking / branch dependency
• UI: Types
• UI: Service methods
• User interface for displaying assignments correctly
• User interface for creating assignments
• Explore UX for creating commits from within a lane

Related to #8637

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