Very simple movie set management - allow set, change and clear

This PR has been created to deal with some frustration with movie sets. Essentially themoviedb dictates what sets are scraped, which is often inconsistent over time as users change the data on tmdb all the time.

This simple solution allows one to clear the associated set, or allocate it to a(nother) set.

Limitations: creation of new sets. Also, when clearing the set of a 1-movie set, the set does no longer list, and the only solution is to re-scrape that one movie.

For the average user, this should be a vast improvement over what's currently there, and allow to rearrange some movies in sets in a more useful way.

@montellese, @jmarshallnz, this is still in the RFC stage, so any feedback welcome.