Refactor V2 version selector so that its logic can be reused

[roluk]

git cannot produce a good diff of accumulated changes because I move the code and files around. I suggest going through each commit individually when doing a thorough review.

• Simplify code by using shorter syntax and making source file tree shallower
• Reformat code to mostly not exceed 100 columns
• Create separation between named version loading state and comparison job tracking state. Make it easier to grasp how each function affects the component state.
• Remove duplicated state. Since the dialog is only open when it's mounted, remove open dialog checks, and use React effect disposal for cancellation signalling.
• Improve cohesion and coupling of useNamedVersionLoading hook so that we can reuse it with minimal impact to future code complexity

The code became easier to read, IMO. If you discover any unwanted behavior, I'll defer fixing it to another PR unless it's a regression.