Fix various typos

Fixes user-facing and non-user-facing typos

Found via `codespell -S "./vendor" -L ontext,rememberable,ridiculus,varius`

app/controllers/admin/roles_controller.rb

@@ -94,7 +94,7 @@ module Admin
       elsif user.add_role @role.name, role_resource
         flash[:notice] = "Successfully added role #{@role.name} to user #{user.email}"
       else
-        flash[:error] = "Coud not add role #{@role.name} to #{user.email}"
+        flash[:error] = "Could not add role #{@role.name} to #{user.email}"
       end
 
       respond_to do |format|

app/controllers/admin/sponsors_controller.rb

@@ -59,7 +59,7 @@ module Admin
       return unless @conference.sponsorship_levels.empty?
 
       redirect_to admin_conference_sponsorship_levels_path(conference_id: @conference.short_title),
-                  alert: 'You need to create atleast one sponsorship level to add a sponsor'
+                  alert: 'You need to create at least one sponsorship level to add a sponsor'
     end
   end
 end

app/controllers/admin/versions_controller.rb

@@ -45,7 +45,7 @@ module Admin
         end
 
       elsif @version.event == 'create' && @version.item
-        # if @version represets a create event and is not currently deleted
+        # if @version represents a create event and is not currently deleted
         @version.item.destroy
         flash[:notice] = 'The selected change was successfully reverted'