Fix multi-guild nickname channel names.

[sebkuip]

This PR does 2 things with channel_format_name.

1. It adds a fallback in the use_nickname_channel_name option where the member wasn't gotten from the guild due to using multi-guild or not cached properly. It will fall back to the user's display name.
2. The ensuring of unique channel names by adding a counter at the end is moved to its own function to reduce cognitive complexity.