Modifying CentralAuthUser->attemptAutoMigration to allow for home wiki specification.

Checks to see if mHomeWiki is set and, if so, does not determine the home wiki
using chooseHomeWiki().  This allows us to set the home wiki in migrateAccount
before running attemptAutoMigration ensuring that the correct account wins.

Change-Id: Ib9364e7a49e3f17f4ca63118b2234d340e862d67
Merge "Modifying CentralAuthUser->attemptAutoMigration to allow for home wiki specification."