Nothing specific to the subscriptions campaign banner, but there are general instructions for styling your discourse instance. I would start there.
It moves it to the right sidebar actually. I don’t know the full history behind it, but probably predates the left-side sidebar being default enabled.
I wouldn’t be apposed to eventually adding some sort of support for a subscribe button/link on the left sidebar though. As an admin though you can customize that section manually and add your own subscribe link.
When you setup a product you can choose which group members will be added to when they subscribe:
There isn’t anything to bump users to a certain trust level, but many settings now have a list of groups to control what they users can do vs just trust levels, so you might be able to add the group you choose to those settings. Just picking a random setting as an example:
EDIT: Actually you can configure your group to bump a user to a certain trust level



