Tidy up your group chats

Whoever starts a group chat can now manage who's in it, plus a handful of fixes for chats and spare invites.

Open the group chat's details and remove anyone who no longer needs to be there. Only the person who started the chat can do it.

Team and group chats list everyone in them again, so @mentions can find every teammate.
Importing players into Invite Spares works again.
The Go to Schedule button after a schedule import is easy to read again.