Change help links in `about:preferences` and menu

Right now, we link to https://mullvad.net/en/help/ in:

• about:preferences under Mullvad Browser Support link at the bottom left
• the menu Help > Get Help

I might be forgetting other places where this link exists.

There's now a dedicated sub-section link we can use instead https://mullvad.net/en/help/tag/mullvad-browser/