Add a Send to Device subview to the page action panel. When the page isn't
sendable, disable the Send to Device menu item. When the user doesn't have any
devices, show a menu item that opens the Firefox Account preferences pane.
Generalize gSync.populateSendTabToDevicesMenu() so that it can be used to
populate any kind of container, not only a menupopup with menuitems.
Add an SVG that shows a phone and an SVG that shows a desktop.
MozReview-Commit-ID: EZQKAEAr08q
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extra : rebase_source : bda87f105712a6c6ba83da1a78179eee93b5f4d0
547921aa65