Fix TagManager
Fix TagManager

It was doing 'find_by' based only on tag, so it was not necessarily finding the right Tag object! Clicking 'delete' on a tag on one course would delete a tag on a different course.

WikiEducationFoundation/WikiEduDashboard,majakomel/WikiEduDashboard,KarmaHater/WikiEduDashboard,Wowu/WikiEduDashboard,WikiEducationFoundation/WikiEduDashboard,MusikAnimal/WikiEduDashboard,alpha721/WikiEduDashboard,WikiEducationFoundation/WikiEduDashboard,alpha721/WikiEduDashboard,KarmaHater/WikiEduDashboard,feelfreelinux/WikiEduDashboard,sejalkhatri/WikiEduDashboard,MusikAnimal/WikiEduDashboard,Wowu/WikiEduDashboard,alpha721/WikiEduDashboard,sejalkhatri/WikiEduDashboard,majakomel/WikiEduDashboard,WikiEducationFoundation/WikiEduDashboard,Wowu/WikiEduDashboard,majakomel/WikiEduDashboard,majakomel/WikiEduDashboard,KarmaHater/WikiEduDashboard,KarmaHater/WikiEduDashboard,feelfreelinux/WikiEduDashboard,MusikAnimal/WikiEduDashboard,Wowu/WikiEduDashboard,feelfreelinux/WikiEduDashboard,feelfreelinux/WikiEduDashboard,WikiEducationFoundation/WikiEduDashboard,sejalkhatri/WikiEduDashboard,sejalkhatri/WikiEduDashboard,sejalkhatri/WikiEduDashboard,alpha721/WikiEduDashboard,MusikAnimal/WikiEduDashboard