R1:4c7370a1a3b0

Make the filetree view width sticky across show/hide and reload

Summary:
Ref T13090. The default width changed recently to become much wider, but the behavior on this control isn't great. Instead:

- Pick a default width somewhere between the two.
- Make the width sticky across show/hide (pressing "f" twice remembers your width instead of resetting it).
- Make the width sticky across reloads (dragging the bar, then reloading the page keeps the bar in the same place).

Test Plan:
- Without settings…