allow for custom min_username_length

change constant in user to be global range and not default

config/locales/server.en.yml

@@ -672,7 +672,9 @@ en:
     invite_only: "Public registration is disabled, new users must be invited"
 
     login_required: "Require authentication to read posts"
-
+    
+    min_username_length: "Minimum username length. (Does not apply if global nickname uniqueness is forced)"
+    
     min_password_length: "Minimum password length."
     block_common_passwords: "Don't allow passwords that are in the 5000 most common passwords."
 

config/site_settings.yml

@@ -93,6 +93,9 @@ users:
   must_approve_users:
     client: true
     default: false
+  min_username_length:
+    client: true
+    default: 3
   min_password_length:
     client: true
     default: 8