Investigate dropping the redirect usage in TrainingController start

Problem/Motivation

TrainingController::start() uses redirects to figure out where to send someone when the click on the learning path 'start/continue' button, however now that this button is in a lazy builder, I wonder whether we could move this logic to the link generation itself and drop the redirects - it would save a double http request for users when they click the button. This logic is not simple though so need to make sure it doesn't significantly cause problems for the link rendering instead.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes