installer: Export wgResourceBasePath in LocalSettingsGenerator
wgRightsIcon uses wgResourceBasePath in its value, therefore it
must be exported first. Otherwise $wgResourceBasePath will still
be null (as from DefaultSettings.php) and cause an incorrect
value to be assigned.
Also fix $wgLogo to use wgResourceBasePath instead of wgScriptPath,
to match what Setup.php does already.