User: Normalize skin key from $wgDefaultSkin for default options

$wgDefaultSkin is allowed not to actually be a valid key (although
doing that is obviously very unwise), which in turn can cause
exceptions from Preferences::loadPreferenceValues()
(MWException "Global default…"), which in turn causes failure of
SpecialPreferencesTest::testBug41337() test.

Change-Id: I8f8ae9ba301337b2e6facd3dcfadbf2ab12ac605
ResourceLoaderModuleTest: Ensure 'vector' is a valid skin

Change-Id: I4c121339aec80a4c56042de7be85cf5f1748b9f0
Merge "User: Normalize skin key from $wgDefaultSkin for default options"
Merge "ResourceLoaderModuleTest: Ensure 'vector' is a valid skin"