[schedule] Cut network and render churn in schedule pages

[frankrousseau]

Problem

• Each bar drag on the team schedule refetched every person's task dates, and re-expanding a row refetched its tasks (2 requests each time)
• On the task type page every task:update socket event rebuilt the whole schedule, refetched all time spents when timesheets are shown, and re-committed the same task extra data
• Entity pages rebuilt their mini schedule (and re-rendered the widget) on any task mutation in the store, even unrelated ones
• The person page filtered and sorted the same task lists three times

Solution

• Recompute the dragged person's root bar locally and cache person tasks per row, invalidated on assign, unassign, drop and date change
• Debounce the socket-driven rebuild (400 ms) and cache the time spents fetch per date range, like the days off
• Commit task extra data only when the reference changed and skip the scheduleItems replacement when the visible signature is unchanged
• Build the merged person task list from the two cached sorted lists

Note: stacked on #2144, only the 7 perf commits are new here.