various ux tweaks

Several different UI cleanups:

 * changed the menu labels for the keybindings to use `⌘` on the mac
 * remove the 'check for updates' menu item. it didn't really do the right thing
 * added an opt-opt for the analytics tracking in the about box
 * removed the no-longer-necessary `navigator.webkitStartDart` code from our bootstrap script
 * changed the files view vertical scroll bar to only show up when necessary