Add Bootstrap listeners to VaadinSession on JmixVaadinServlet initialization

[glebfox]

For QA:

Add custom BootstrapListener that modifies the main page, e.g.:

import com.vaadin.server.BootstrapFragmentResponse;
import com.vaadin.server.BootstrapListener;
import com.vaadin.server.BootstrapPageResponse;
import org.jsoup.nodes.Element;
import org.springframework.stereotype.Component;

@Component(CustomBootstrapListener.NAME)
public class CustomBootstrapListener implements BootstrapListener {

    public static final String NAME = "demo_BootstrapListener";

    @Override
    public void modifyBootstrapFragment(BootstrapFragmentResponse response) {
    }

    @Override
    public void modifyBootstrapPage(BootstrapPageResponse response) {
        Element head = response.getDocument().getElementsByTag("head").get(0);

        Element meta = response.getDocument().createElement("meta");
        meta.attr("name", "viewport");
        meta.attr("content", "width=device-width, initial-scale=" + 0.8);
        head.appendChild(meta);
    }
}

Check result: