refs https://github.com/TryGhost/Team/issues/2677
When doing CMD+TAB, then CMD+TAB+shift, the CMD key got released but both the keyup and blur events weren't called (also focus not).
To fix this, on the next click we'll check the meta, shift and ctrl keys so we can restore the situation.
fixes https://github.com/TryGhost/Team/issues/2983
When navigating, the posts selection will get cleared. This also fixes a bug with CMD+a that had an old event listener that was not cleared correctly, breakign CMD+A after navigation.
fixes https://github.com/TryGhost/Team/issues/2939
When using CMD+F, the multiple selection list still thinks CMD is pressed because the keyup event is not fired. This is fixed by clearing the pressed keys when the window is blurred.
refs https://github.com/TryGhost/Team/issues/2906
Adds a way to select posts using CMD, shift and CMD+A. And adds a placeholder context menu.
Behind the making it rain feature flag.
