Add forum link to Help menu and improve user login/register flow

1. In the Help menu, we miss the link to the forum which allows people to ask a question: "Ask for help"

2. When clicking on the Report a bug in the Help menu, you end up on https://musescore.org/en/node/add/project-issue/musescore. In case you are not logged in, you get an access denied. Push users instead to the login/register form.

Solution

To fix both at once, a new endpoint need to be created which does the appropriate action.