Add 'wmgUniversalLanguageSelectorDefault'

- Adds 'wmgUniversalLanguageSelectorDefault' configuration variable.
- When true, $wgDefaultUserOptions['uls-enable'] is set to 1, meaning ULS is
  enabled by default unless explicity disabled by user.
- wmgUniversalLanguageSelectorDefault defaults to false
- True on wikidatawiki, wikidatatestwiki, and testwiki.

Change-Id: Ie9ff567554737333a6a97130f94a6a0592837914