Deprecate current 3 day preview trial

[justinrobots]

We're replacing the current 3 day preview trial with a new feature preview (see parent issue).

The entry points into the existing 3 day preview trial should be removed, but the system should remain in place in the case we need to bring it back.

See @eamodio or @d13 for technical clarification or questions.

[d13]

For existing pro features, we only want to remove the ability to start a preview trial in the UI, aka in the webview gates and the view welcome screens. That way, we don't need to remove any account state logic.