Hide unused tabs

Running with @kopareigns' idea to hide tabs that are "empty" of slaves (!4034 (merged)), but there was a snag.

Imagine there is a single slave whoring, and you click on them via the tab. From slave interact, you assign them a different job, then return to main. Boom, error, because it's trying to load the most recent tab, whores, but the whore tab no longer exists.

I covered this edge case by checking length before choosing a tab.

I have left the "rest" tab always on since it acts as a catch all for coming and going slaves and I like to leave it up myself from week to week, even if empty.