Do not remove torrents when their categories are removed and allow torrents without categories

[mickvandijke]

In the current behaviour, when deleting existing categories, all torrents in that category will be removed. Instead we should move the torrents to a new permanent category called Uncategorized.

[josecelano]

Today we agreed on removing the category from the torrents and allowing torrents without categories.

cc @WarmBeer @da2ce7