UX: Introduce automatic 'categories topics' setting (#8804)

When 'categories topics' setting is set to 0, the system will
automatically try to find a value to keep the two columns (categories
and topics) symmetrical.

The value is computed as 1.5x the number of top level categories and at
least 5 topics will always be returned.

lib/validators/categories_topics_validator.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+class CategoriesTopicsValidator
+  def initialize(opts = {})
+    @opts = opts
+  end
+
+  def valid_value?(val)
+    num = val.to_i
+    return false if num.to_s != val.to_s
+    return false if num != 0 && num < CategoriesController::MIN_CATEGORIES_TOPICS
+
+    true
+  end
+
+  def error_message
+    I18n.t('site_settings.errors.invalid_integer_min', min: 5)
+  end
+end