Add campaign support

?campaign=someName in the query string of the Create account form
sends JavaScript to client that logs a dbName-campaign session cookie.

Upon successful account creation, it logs this campaign
cookie as part of the ServerSideAccountCreation event.

See https://www.mediawiki.org/wiki/Extension:Campaigns

The corresponding commit that removes the 'AddNewAccount' code from
EventLogging.hooks.php is Ic6e8f24e6d758309d201dc95c16016e823943a5a

PS 1 implemented logging and cookie in client-side JavaScript.
PS 2 implemented logging and cookie in server-side PHP, including
     Matt Flaschen's reimplementation of mw.user.sessionId() token.
PS 4 only set session cookie in PHP.
PS 5 set cookie on client; ServerSideAccountCreation logging moved here
     from Extension:EventLogging.

Change-Id: Ife949daf51210d68a3328fc7b4bfef581afe80cd