Allow more than one tag in argument in get_tags().

Because dynamic input fields are not implemented in the Web-UI, we use
one input field with commas for now.

src/cgi.cpp

@@ -123,7 +123,10 @@ vector<string> get_tags()
         const string tag{element.getValue()};
         if (!tag.empty())
         {
-            tags_string.push_back(tolower(tag));
+            const auto new_tags{string_to_vector(tolower(tag))};
+            tags_string.insert(tags_string.end(),
+                               std::make_move_iterator(new_tags.begin()),
+                               std::make_move_iterator(new_tags.end()));
         }
     }