Prevent / being used in bot or server prefixes (#5693)

* Update `[p]set prefix`/`[p]set serverprefix`

* Update cli

* style

* update __main__

* style

* improve checks

* Raise in Red.set_prefixes, update responses

* uniform responses

* Fixes

* Keep generator variable names consistent across files

redbot/core/cli.py

@@ -70,6 +70,11 @@ async def interactive_config(red, token_set, prefix_set, *, print_header=True):
             if len(prefix) > 10:
                 if not confirm("Your prefix seems overly long. Are you sure that it's correct?"):
                     prefix = ""
+            if prefix.startswith("/"):
+                print(
+                    "Prefixes cannot start with '/', as it conflicts with Discord's slash commands."
+                )
+                prefix = ""
             if prefix:
                 await red._config.prefix.set([prefix])