Optimise WikiPage::newFromID() to load all the data in one database query.

Optimise WikiPage::newFromID() to load all the data in one database query.

* Separated the last part of WikiPage::loadPageData() into loadFromRow() to put database access and object loading into separate methods (and also for consistency with other classes)
* Added WikiPage::newFromRow() to be able to get a WikiPage object from an already-existing database row
* Modified WikiPage::newFromID() to do a single database request to get all the necessary data and make it use the new newFromRow() method

Change-Id: I2db423f2dba10cc3db4f4b2e7a7b9d99cd114f33