FEATURE: make username suggester suggest user1, user2 etc. for input that contains invalid characters only (#14179)

We were suggesting 111, 1111, 1112 before.

See the discussion on Meta – https://meta.discourse.org/t/curious-account-creation-behaviour/199970/14.

lib/user_name_suggester.rb

@@ -2,6 +2,7 @@
 
 module UserNameSuggester
   GENERIC_NAMES = ['i', 'me', 'info', 'support', 'admin', 'webmaster', 'hello', 'mail', 'office', 'contact', 'team']
+  LAST_RESORT_USERNAME = "user"
 
   def self.suggest(name_or_email, allowed_username = nil)
     return unless name_or_email.present?
@@ -102,7 +103,13 @@ module UserNameSuggester
   end
 
   def self.fix_username(name)
-    rightsize_username(sanitize_username(name))
+    fixed_username = sanitize_username(name)
+    if fixed_username.empty?
+      fixed_username << sanitize_username(I18n.t('fallback_username'))
+      fixed_username << LAST_RESORT_USERNAME if fixed_username.empty?
+    end
+
+    rightsize_username(fixed_username)
   end
 
   def self.sanitize_username(name)