First version of Page Language selector

Special page PageLanguage to set the page language of a page.
To enable the feature, set $wgPageLanguageUseDB to true
and assign the 'pagelang' user right to a user group.

Bug: 35489
Change-Id: I0f82b146fbe948f917c1c5d29f7469644d797e80
Merge "First version of Page Language selector"