Change update_responses URL to include a version number

I see we currently have this as update URL:

• https://cdn.mullvad.net/browser/update_responses/%CHANNEL%/%BUILD_TARGET%/%VERSION%/ALL

I think https://cdn.mullvad.net/browser/update_responses/ will be the root of the updates_responses git repository (since I assume https://cdn.mullvad.net/browser/ will also contain the mar files), so we have the %CHANNEL% directory directly at the root of the repository, which will make it more difficult to change the URL later (for watershed update for example).

So I think we should change the update URL to something like this:

• https://cdn.mullvad.net/browser/update_responses/update_1/%CHANNEL%/%BUILD_TARGET%/%VERSION%/ALL

so we can change the update_1 to something else later, if needed.

To make the update from the current version work, we can add some symlink release -> update_1/release.