postEdit: Only call mw.cookie.get() if needed

postEdit: Only call mw.cookie.get() if needed

Avoid synchronous document.cookie reads unconditionally during page load.
At least avoid the read where it isn't needed, such as non-view actions
and special pages.

Ideally this would happen in mw.requestIdleCallback, but unfurtunately
various extensions have a fragile dependency on reading mw.config
wgPostEdit at initialitation time which would likely fail if this is no
longer set synchronously. To be revisited...

Change-Id: Ib655bf6507333c1a9845a05eb436dca522efbab7