Add support for the "(page does not exist)" tooltip
Add support for the "(page does not exist)" tooltip

Depends on change Ia76dd6db363f6add5efb8955be9e23a1f8e8476f
in MediaWiki, which adds support for the 'exists' option.
(This change doesn't do anything on older versions.)

Also change the check from Title::isKnown() to Title::exists().
This is consistent with how we decide whether to link to action=edit
above, and the user sandbox is never "always known" anyway.

Change-Id: I4805916fb7f3b22eb5ad0e6971bccc238f17ea68

wikimedia/mediawiki-extensions-SandboxLink,wikimedia/mediawiki-extensions-SandboxLink