This function handles the ins and outs of the screen for adding new ban
triggers or modifying existing ones.
Adding new ban triggers:
- is accessed by ?action=admin;area=ban;sa=edittrigger;bg=x
- uses the ban_edit_trigger sub template of ManageBans.
Editing existing ban triggers:
- is accessed by ?action=admin;area=ban;sa=edittrigger;bg=x;bi=y
- uses the ban_edit_trigger sub template of ManageBans.