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

spec/lib/seed_data/categories_spec.rb

@@ -107,6 +107,7 @@ RSpec.describe SeedData::Categories do
         .and change { Topic.count }
 
       expect(Category.last.name).to eq("General")
+      expect(SiteSetting.default_composer_category).to eq(Category.last.id)
     end
 
     it "adds default categories SiteSetting.default_sidebar_categories" do