MDL-33774 mod_label stores the content where it is supposed to

removed hardcoded checking for 'label' module type
MDL-33774 instead of checking that activity is label we check that it has view url
MDL-33774 filter_activitynames removed hardcoded label module type
MDL-33774 Converting static cache to MUC in filter_activitynames

Static cache was breaking consequtive PHPunit tests after modifications made in MDL-33774
removed caching in filter_activitynames
MDL-33774 get rid of cache in global var