SECURITY: Recheck home wiki when user merges accounts (1.24wmf22)

We check that the user can authenticate to the chosen home wiki when
doing the migration dry-run, but don't recheck when doing the actual
migration.

Since storeAndMigrate is used by migration scripts too, add a flag to
the function to signal when we need to recheck owndership.

Bug: 70468
Change-Id: I3574abe7d1f2ccb6d1241e07c900defde4acde8e
SECURITY: Check ownership of homewiki when globalizing on login

If the person logging in didn't own the homewiki, they could still
get attached to it. They wouldn't be able to do anything malicious,
since they wouldn't have the correct password, but would be locked
out of their account upon next login.

Bug: 71749
Change-Id: I2928795d405d1a6b7a126fd4dc4f47cb409a5439