(proxy) extend number of scrollbar itesm, closes https://github.com/opnsense/core/issues/819

src/opnsense/mvc/app/controllers/OPNsense/Proxy/forms/dialogEditBlacklist.xml

@@ -23,6 +23,7 @@
         <type>text</type>
         <type>select_multiple</type>
         <style>tokenize</style>
+        <nbDropdownElements>300</nbDropdownElements>
         <help><![CDATA[select categories to use, leave empty for all. Categories are visible after initial download.]]></help>
     </field>
     <field>