FIX: Properly encoded slugs when configured to (#8158)

When an admin changes the site setting slug_generation_method to
encoded, we weren't really encoding the slug, but just allowing non-ascii
characters in the slug (unicode).

That brings problems when a user posts a link to topic without the slug, as
our topic controller tries to redirect the user to the correct URL that contains
the slug with unicode characters. Having unicode in the Location header in a
response is a RFC violation and some browsers end up in a redirection loop.

Bug report: https://meta.discourse.org/t/-/125371?u=falco

This commit also checks if a site uses encoded slugs and clear all saved slugs
in the db so they can be regenerated using an onceoff job.

spec/components/slug_spec.rb

@@ -60,7 +60,7 @@ describe Slug do
       after { SiteSetting.slug_generation_method = 'ascii' }
 
       it 'generates the slug' do
-        expect(Slug.for("熱帶風暴畫眉")).to eq('熱帶風暴畫眉')
+        expect(Slug.for("熱帶風暴畫眉")).to eq('%E7%86%B1%E5%B8%B6%E9%A2%A8%E6%9A%B4%E7%95%AB%E7%9C%89')
         expect(Slug.for("Jeff hate's !~-_|,=#this")).to eq("jeff-hates-this")
       end
 
@@ -75,7 +75,7 @@ describe Slug do
       it "handles the special characters" do
         expect(Slug.for(
           " - English and Chinese title with special characters / 中文标题 !@:?\\:'`#^& $%&*()` -- "
-        )).to eq("english-and-chinese-title-with-special-characters-中文标题")
+        )).to eq("english-and-chinese-title-with-special-characters-%E4%B8%AD%E6%96%87%E6%A0%87%E9%A2%98")
       end
 
       it "kills the trailing dash" do
@@ -151,9 +151,9 @@ describe Slug do
     after { SiteSetting.slug_generation_method = 'ascii' }
 
     it 'generates precentage encoded string' do
-      expect(Slug.encoded_generator("뉴스피드")).to eq("뉴스피드")
-      expect(Slug.encoded_generator("آموزش اضافه کردن لینک اختیاری به هدر")).to eq("آموزش-اضافه-کردن-لینک-اختیاری-به-هدر")
-      expect(Slug.encoded_generator("熱帶風暴畫眉")).to eq("熱帶風暴畫眉")
+      expect(Slug.encoded_generator("뉴스피드")).to eq("%EB%89%B4%EC%8A%A4%ED%94%BC%EB%93%9C")
+      expect(Slug.encoded_generator("آموزش اضافه کردن لینک اختیاری به هدر")).to eq("%D8%A2%D9%85%D9%88%D8%B2%D8%B4-%D8%A7%D8%B6%D8%A7%D9%81%D9%87-%DA%A9%D8%B1%D8%AF%D9%86-%D9%84%DB%8C%D9%86%DA%A9-%D8%A7%D8%AE%D8%AA%DB%8C%D8%A7%D8%B1%DB%8C-%D8%A8%D9%87-%D9%87%D8%AF%D8%B1")
+      expect(Slug.encoded_generator("熱帶風暴畫眉")).to eq("%E7%86%B1%E5%B8%B6%E9%A2%A8%E6%9A%B4%E7%95%AB%E7%9C%89")
     end
 
     it 'reject RFC 3986 reserved character and blank' do