Add a cog to manage tracking a user's alt accounts

[ChrisLovering]

Important note

This must be merged after https://github.com/python-discord/site/pull/1171

Summary

This allows moderators, in mod channels, to add, remove & list a user's alt accounts.

When searching for infractions by user id, the number of al accounts is also shown, if there are any.

Screenshot

Wording can be discussed...