Hygiene: Merge side bar menu in SkinMobile into SkinMinerva

Add FIXME notes saying that SkinMobile and MobileTemplate should be
removed and make clearer the purpose of SkinMobile and MobileTemplate

To test add to LocalSettings.php
$wgValidSkinNames['minerva'] = "Minerva";
$wgDefaultSkin = 'minerva';
and load desktop site

Change-Id: If9359a6e56147686c16aa34e99cdd0b40de89efd
Merge "Hygiene: Merge side bar menu in SkinMobile into SkinMinerva"