FIX: do not allow tag with name 'none' (#9867)

https://meta.discourse.org/t/none-tag-is-uneditable/152003

db/migrate/20200525072638_remove_none_tags.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+class RemoveNoneTags < ActiveRecord::Migration[6.0]
+  def up
+    none_tag_id = DB.query_single("SELECT id FROM tags WHERE lower(name) = 'none'").first
+    if none_tag_id.present?
+      [:tag_users, :topic_tags, :category_tag_stats, :category_tags, :tag_group_memberships].each do |table_name|
+        execute "DELETE FROM #{table_name} WHERE tag_id = #{none_tag_id}"
+      end
+      execute "DELETE FROM tags WHERE id = #{none_tag_id} OR target_tag_id = #{none_tag_id}"
+    end
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end