FEATURE: Default Composer Category Site Setting (#18967)

* FEATURE: Default Composer Category Site Setting

- Create the default_composer_category site setting
- Replace general_category_id logic for auto selecting the composer
  category
- Prevent Uncategorized from being selected if not allowed
- Add default_composer_category option to seeded categories
- Create a migration to populate the default_composer_category site
  setting if there is a general_category_id populated
- Added some tests

* Add missing translation for the new site setting

* fix some js tests

* Just check that the header value is null

lib/validators/default_composer_category_validator.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+class DefaultComposerCategoryValidator
+  def initialize(opts = {})
+    @opts = opts
+  end
+
+  def valid_value?(val)
+    category_id = val.to_i
+    unless SiteSetting.allow_uncategorized_topics
+      return false if category_id == SiteSetting.uncategorized_category_id
+    end
+    true
+  end
+
+  def error_message
+    I18n.t('site_settings.errors.invalid_uncategorized_category_setting')
+  end
+end