Add character sheet section to display summoned creatures details

[arbron]

It would be nice to have a way on the character sheet to track any creatures summoned by that character with easy controls for opening their sheet and un-summoning them.

This could perhaps eventually be expanded into a whole separate tab that appears when creatures are summoned that displays a version of those creature's stat blocks to allow players to play their summons from their character sheet.

• [x] Add static summoned creature registry similar to what was implemented for enchantments
• [ ] Display summoned creatures on item that summoned them
• [ ] Display summoned creatures on character sheet for actor who summoned them
• [ ] (Maybe) Add separate tab with simple version of summoned creature's sheet