FIX: Make category slugs lowercase (#11277)

Admins could specify category slug with upper case characters and same slug,
but with different cases could be used simultaneously.

app/models/category.rb

@@ -343,8 +343,7 @@ class Category < ActiveRecord::Base
     if slug.present?
       # if we don't unescape it first we strip the % from the encoded version
       slug = SiteSetting.slug_generation_method == 'encoded' ? CGI.unescape(self.slug) : self.slug
-      # sanitize the custom slug
-      self.slug = Slug.sanitize(slug)
+      self.slug = Slug.for(slug, '', method: :encoded)
 
       if self.slug.blank?
         errors.add(:slug, :invalid)
@@ -795,8 +794,12 @@ class Category < ActiveRecord::Base
     return nil if slug_path.empty?
     return nil if slug_path.size > SiteSetting.max_category_nesting
 
-    if SiteSetting.slug_generation_method == "encoded"
-      slug_path.map! { |slug| CGI.escape(slug) }
+    slug_path.map! do |slug|
+      if SiteSetting.slug_generation_method == "encoded"
+        CGI.escape(slug.downcase)
+      else
+        slug.downcase
+      end
     end
 
     query =