Increase priority of Bound and Summon spells

Fixes #2623 (closed) by basically just doing what @Kuyondo suggested, with the minor addition of not letting the AI cast multiple bound weapons.

Note that this doesn't actually match vanilla behaviour. Morrowind.exe will happily make the AI try and cast a 6 magicka summon while it has next to no chance of doing so successfully, even if it has a 300 magicka spell it can cast consistently. Which is so similar to #4207 (closed) that I don't think we should replicate it faithfully. Still, rating summons higher makes plenty of sense.