Implement "attached_image_max_height" and "attached_image_show_default" config param handling. A new block "head" is added to wrapper_document.tx so that the child template can insert its custom <style> tag.
t/*: refactor creating BusyBird::Main for test into testlib::Main_Util::create_main(). This was repeated in so many test scripts.
Main: fix default value for "attached_image_show_default" param.
Add t/Main_PSGI_View_config.t, which tests config-dependent view elements.
Main, Main::PSGI::View: implement "attached_image_urls_builder" config parameter. Pass the test.