fix(Opponent Selection): Fix a bug where characters do not reappear for selection after being unselected

fix(Opponent Selection): Fix a bug where characters do not reappear for selection after being unselected.

What was going on was that we remove a character from the list of those selectable after it is selected, but never return it to the list if it is unselected. This doesn't manifest if you select a character and then remove it before clicking on another empty slot because the check and removal occurs when we click an empty slot before the selectable characters are displayed, rather than on the select itself (so the character isn't selected at the time we run the check). Similar bugs existed wherein clicking Remove All or trying to start a new game with the same opponents just after finishing one caused all characters except for the very last one selected to disappear, for the same reason. The solution is just to make sure we are filtering from a fresh list every time we open the character selection screen, rather than filtering from a copy of the list we've already filtered, thus ensuring removed opponents are properly re-added to the list whenever they should be. Tested locally in FireFox.

Let me know if there's anything else you'd like me to do to get this merge-ready.