Refactor and fixes for the Party Sheet

[freohr]

• Replaces the actor selection modal with drag&drop from the sidebar (as well as a small button to remove Actor from the Party).
• Enable dragging actors from the Party Sheet (e.g. to put them on a scene)
• Fix the rendering synchronization of the party sheet when updating who is in the party (on all currently connected clients)
  • Which means that the now obsolete "sync" button has been removed
• Small refactor of the XP modal
  • Auto calculate XP shares on input (instead of having to click on the button, which has been removed)
  • Close the XP modal on submit

Also, if we ever put up the site with the quick overview of the system, the part about the Party Overview has been updated to match the changes done in this PR.

P.S.: Sorry about the full reformat of the help page, I just manually saved the page in VSCode which triggered the reformat >.>

[anthonyronda]

Sorry about the full reformat of the help page, I just manually saved the page in VSCode which triggered the reformat >.>

No worries! Reviewing now

[anthonyronda]

LGTM. Works well in testing!