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, loaded page: got medium-width bar.
• Dragged bar wide/narrow, toggled on/off with "f", got persistent width.
• Dragged bar wide/narrow, reloaded page, got persistent width.
• Dragged bar wide/narrow, toggled it off, reloaded page, toggled it on, got persistent width.

Maniphest Tasks: T13090

Differential Revision: https://secure.phabricator.com/D19129