Support multiplayer dual-team hp calculation

[Wes Chao]

I'm envisioning a checkbox for "coop dungeon"; when clicked, it reveals a new section, "Friend Team". There would initially be a team ID entry box, just like the existing one. Entering a team ID would trigger logic to check that the two leads are the same, displaying an error if not, then load an abbreviated version of the team table into the Friend Team section. The HP total should then be updated to be the total of both teams' subs, plus the leads (not the sum of both teams' hp, since that double-counts the leads).

It's kind of a lot of work, but the hp total is really useful, especially since a number of the coop dungeons feature hard-hitting monsters that you need both teams' hp to survive. Hopefully we can just reuse a bunch of code to make it work.