FIX: If a prettified slug is a number, return defaultt (#8554)

Meta thread: https://meta.discourse.org/t/sending-a-pm-with-the-following-title-causes-an-error/135654/3

We had an issue where if someone sent a PM with crazy
characters that are stripped and we end up with only
a number, the topic redirect errored because the slug was
a number. so instead we return the default as well if
the slug is a number after prettification

spec/components/slug_spec.rb

@@ -17,6 +17,10 @@ describe Slug do
       expect(Slug.for('')).to eq default_slug
     end
 
+    it 'return topic by default if the string boils down to a number' do
+      expect(Slug.for('=213=-!(@#+@)(!*_(@#&(!)#')).to eq default_slug
+    end
+
     it 'accepts fallback' do
       expect(Slug.for('', 'king')).to eq 'king'
     end