How viewable should deleted users be by moderators?

[trichoplax]

Describe the bug
The user profile page of a deleted user shows no information to a standard user (as expected), but to a moderator shows the profile information accompanied by a message:

This profile is deleted. You have access to this page as a moderator, but this is not public information.

Moderators therefore have access to this information, but no obvious way to find the page. When a user is deleted, links to the user profile page are removed for standard users (as expected), but they are also removed for moderators. For example, all of the following places do not let a moderator navigate to the deleted user's profile page:

• The list of users (deleted users do not show up, with no option for a moderator to view them).
• The "Suggested by" section of a suggested edit (the user shows as "deleted user" and the link goes back to the same page rather than the profile page).
• The username of a comment (the user shows as "deleted user" and the link goes back to the same page).
• The user card below a post (this shows as "deleted user" with no link).

This means that a moderator can find out which user was the author in all of these cases, but only by an inconvenient process of changing URLs. Would it be better to be consistent, and do one of the following:

• Deny the moderator the information altogether (by making the profile page unviewable).
• Make it easy for the moderator to see information that they are allowed to see, by making the links to the profile still work for moderators after deletion.

What access to deleted user information should each of the following have:

• User
• Curator
• Moderator
• Admin
• Developer

[cellio]

Related (on the non-functional links): #1089

[cellio]

It's important for moderators (and admins) to be able to see deleted users with their associated activity, for investigations into serial spammers/trolls/etc.

I would like user links to deleted users to behave like their non-deleted cousins for moderators, to make it easy to get to those profiles from anywhere the user left activity behind. And I would like for them to not be links at all for people who can't see the profiles (discussed in the related bug I just added).

As for who, at least moderators and admins (and developers) should be able to see these profile pages and links, and it's possible we should broaden that to curators. I assume we'll add one function, something like can_see_deleted_users, and put the logic in there, so it'd be easy to broaden the permissions later (start with just mod/admin/dev).

Separately, and after the new mod tools merge, let's consider adding a "show deleted users" option somewhere, like mods have for deleted posts.