LUN-625 -- blog river templates validation
LUN-625 -- code review changes
LUN-625 -- added validation for showmenu tag templates
LUN-625 -- created hook to validate plugins templates + show_menu tag unit test
LUN-625 -- unit test for testing show_menu_tag + extracting common code to use in lunchbox tests
LUN-625 -- renaming variable
LUN-625 -- fixed comment
LUN-625 -- transform the list into a set
LUN-625 -- its a set already
LUN-625 -- no need for Pretty Name in PLUGIN_TEMPLATE_REFERENCES since I can get it from plugin_cls.name
Merge pull request #13 from pbs/LUN-625

LUN-625 --It shouldn't be possible to unassign a template from a site if that template exist within that site(is used by menu or other plugin)