FIX: Do not truncate encoded slugs

Trying to truncate encoded slugs will mean that we have to keep the URL
valid, which can be tricky as you have to be aware of multibyte
characters.

Since we already have upper bounds for the title, the slug won't grow
for more than title*6 in the worst case. The slug column in the topic
table can store that just fine.

Added a test to ensure that a generated slug is a valid URL too, so we
don't introduce regressions in the future.

lib/slug.rb

@@ -9,6 +9,10 @@ module Slug
   def self.for(string, default = 'topic', max_length = MAX_LENGTH)
     string = string.gsub(/:([\w\-+]+(?::t\d)?):/, '') if string.present? # strip emoji strings
 
+    if SiteSetting.slug_generation_method == 'encoded'
+      max_length = 9999 # do not truncate encoded slugs
+    end
+
     slug =
       case (SiteSetting.slug_generation_method || :ascii).to_sym
       when :ascii then self.ascii_generator(string)