Vault refinements and notification fixes

A round of Vault refinements, along with fixes to the release notification system.

Fixes

Reliable folder previews and safer moves. Folder previews now render correctly, and a folder can no longer be moved into itself.
Clearer Vault folders and menus. Folders now show their file and subfolder counts in both list and grid views, the file menu lists only the actions that apply to each file, and list columns hold a steady width as you move between items.