Implement option to prevent duplicate joins

In thinking about rewriting a reporting tool we built to be more QB-focused instead of ORM-focused, we want to be able to dynamically construct both query/filter targets (FROMs, WHEREs, JOINs) and display targets (SELECTs) using QB syntax.

This is very easy, but if we select a half-dozen elements that all rely on the same join, we'd have to build in a lot of application logic to figure out whether we already asked for the join in question to avoid joining the same table multiple times.

After a brief Slack discussion, we worked on a PR to add this capability to QB without breaking any existing behavior. This task is a placeholder for that PR and any related discussion.