Since an Angular digest cycle is triggered whenever Redux store state
changes, we can simplify the top-level component controllers in the UI
by making `vm.rootThread` a method which generates the top-level thread
when called.

Since `rootThread.thread` memoizes based its input, thread recalculation
won't happen more often than necessary.

This change should also make it easier to migrate these components to
Preact/React in future.