s/redirect_to :back/redirect_back/

`redirect_to` has been removed in Rails 5.1 in favor of:

```
redirect_back(fallback_location: root_path)
```

app/controllers/admin/commercials_controller.rb

@@ -70,7 +70,7 @@ module Admin
 
         flash[:error] = errors_text
       end
-      redirect_to :back
+      redirect_back(fallback_location: root_path)
     end
 
     private

app/controllers/openids_controller.rb

@@ -6,6 +6,6 @@ class OpenidsController < ApplicationController
 
   def destroy
     @openid.destroy
-    redirect_to :back
+    redirect_back(fallback_location: root_path)
   end
 end

app/controllers/proposals_controller.rb

@@ -104,7 +104,7 @@ class ProposalsController < ApplicationController
         @event.event_schedules.destroy_all
       end
     rescue Transitions::InvalidTransition
-      redirect_to :back, error: "Event can't be withdrawn"
+      redirect_back(fallback_location: root_path, error: "Event can't be withdrawn")
       return
     end
 
@@ -124,7 +124,7 @@ class ProposalsController < ApplicationController
     begin
       @event.confirm
     rescue Transitions::InvalidTransition
-      redirect_to :back, error: "Event can't be confirmed"
+      redirect_back(fallback_location: root_path, error: "Event can't be confirmed")
       return
     end
 

app/controllers/surveys_controller.rb

@@ -40,6 +40,6 @@ class SurveysController < ApplicationController
       end
     end
 
-    redirect_to :back
+    redirect_back(fallback_location: root_path)
   end
 end

app/helpers/application_helper.rb

@@ -88,10 +88,9 @@ module ApplicationHelper
     end
   end
 
-  # Same as redirect_to(:back) if there is a valid HTTP referer, otherwise redirect_to()
   def redirect_back_or_to(options = {}, response_status = {})
     if request.env['HTTP_REFERER']
-      redirect_to :back
+      redirect_back(fallback_location: root_path)
     else
       redirect_to options, response_status
     end