OB6: Show a popup CTA to onboarders in test group

global $wgGettingStartedRunTest=false disables test
Restore isInTest() for even userIds, export to JavaScript.

Rename the current test JS to showSeparatePage as it's now the "control"
behavior.

If in test, let CentralAuth/SUL2 redirect the new user to the returnTo
page, but add gettingStartedReturn=true.  On every page load,
check that flag and if present, load new return.js, which detects
the four kinds of article pages per the flowchart, and for some kinds
shows a Call To Action dialog built with jQuery UI dialog().  Handling the CTA
reuses existing code for openTask cookie, task toolbar, find copyediting
tasks, launch 'firstedit' or 'firsteditve' tour if user edits current
page

Version 6 logging against new Schema:GettingStartedOnRedirect.

Unrelated fix in SpecialGettingStarted.php to not crash if wfFindFile()
fails because network is down.

Use wgRevisionId so old revs work properly.

Don't use addModules with mediawiki.ui.

https://www.mediawiki.org/wiki/Extension:CentralAuth explains how to
configure your wiki to get the SUL2 redirect.

Require CentralAuth silent login flow and simplify code accordingly.

Co-Author: S Page <spage@wikimedia.org>
Co-Author: Matthew Flaschen <mflaschen@wikimedia.org>

Change-Id: Idae2ec87d43759f436e764564ae4569d065a832a
Merge "OB6: Show a popup CTA to onboarders in test group"