Allow users to hide community membership and mutual info

[du82]

As a user, I want to be able to hide my existence in a community, as well as hiding "mutual friends" and "mutual communities" to prevent relationship mapping attacks.

Discord membership relationship mapping attacks

Discord has a problem which directly utilizes the "mutual friends" and "mutual servers" information in a profile, as well as the sidebar in a community. This data can be used to build a full map of what a user does on Discord, without having access to their token or account.

This is not some far-fetched theoretical attack. It is easily carried out with as little as two accounts, and it happens thousands of times every day by abusing the APIs, and moreso now the GUI to gather large amounts of up-to-date info on people.