mediawiki.page.ready: Avoid duplicate DOM query on logout click

mediawiki.page.ready: Avoid duplicate DOM query on logout click

The result of this DOM selector query is already available
as 'this' given we're in an event handler, use that directly
instead.

Also:
* Use the 'href' property directly instead of using jQuery or attributes.
* Avoid shadow names by naming the error handler param 'err' instead
  of 'e'.

Change-Id: I94a757ca1dd782b1e138a372983b6bfa16081485