Add a "party" feature

[DominicEliot]

The idea is that every player character will be a member of a party, and that's where things like supply will be kept track of.

Things to figure out: [ ] How does someone add or remove an entity from the party? [ ] Should the player's ship be tracked in the party? [ ] Should companions be tracked in the party? [ ] Should the party take over for supply on Player cards? [ ] What about if there's only one player in the party? [ ] How does this impact things like the /action command?

[rsek]

here's an outline of a basic command implementation that could get us started:

• all PCs default to being a party of one
• party membership is a required parameter when generating a PC with the slash command (i think prompting people with it is a good idea to expose the functionality to them)
  • the parameter is an autocomplete parameter; it always sends back an option of "None/Solo", at minimum
  • it also returns all existing Party objects
  • "None/Solo" simply leaves the PC in their party of one
  • selecting another PC in a party of one adds the new PC to that party
  • parties of multiple PCs can list out their various PCs, like: "Party of PC1, PC2, PC3"; autocomplete should check against the names of each party member.
  • if a command vehicle is attached to a party, its name could be used to autocomplete too, and included in the text result: "Party of PC1, PC2, PC3 (Starship Name)"

with that in place, we could look at ways to keep Supply synced across PCs (which i think should be the default behaviour). unsyncing Supply could simply be a question of leaving a party.

PCs could also be added to parties with a slash command after character creation. parameter 1 would be the PC to add to a party, and its autocomplete would return all PCs that are currently a party of one. parameter 2 would return all Party objects (so a PC could be added to another party of one to form a new party).