Default $wgActorTableSchemaMigrationStage to READ_NEW

Default $wgActorTableSchemaMigrationStage to READ_NEW

Probably good to start testing this in CI sometime soon.

The unit tests being changed here are:
* Hacky importing of a revision in MessageCacheTest, changed to use an
  interwiki username.
* WikiPageDbTestBase will get null rather than 0 for ar_user.
* DumpTestCase was using a custom IP $wgUser when creating its test
  revisions, which was winding up referring to an actor_id that had been
  deleted from the database. Fixed by having it use a registered user
  instead.

Also this has MediaWikiTestCase reset the $wgUser User object when the
actor table is truncated, as various tests were otherwise failing because
it was caching a later-deleted actor ID for User:127.0.0.1.

Bug: T188327
Change-Id: Ic483d0fda931810ac639b9de1cc5e5a4ef8161cc
Depends-On: I1bd0288cc132627d75b4001219522ec5e952eda7