AO3-6672 Allow some admins to set a generic username

[brianjaustin]

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Issue

https://otwarchive.atlassian.net/browse/AO3-6672

Purpose

Allow superadmins and policy_and_abuse to set a generic username for users (even when banned). Because #4991 is already open, I have not added the subscript for "Ticket ID" as noted in the ticket.

Side question: do we want to update some of the copy/variables here to be more consistent? I observed login (has to stay), user name, and username in different spots...

[sarken]

If you're up for changing copy in a whole bunch of places, I'm up for testing! We ultimately want to switch from "user name" to "username" but it's in soooo many places I didn't want to inflict that on anyone.

I think this covers all the references to this page so we could at least have some consistency in this area:

• this change_username page, including error messages
• the link in the /views/users/_edit_header_navigation.html.erb partial
• the link in the /views/users/_edit_user_navigation.html.erb partial
• the reference to this page on /app/views/pseuds/_pseuds_form.html.erb ("You cannot change the pseud that matches your user name. However, you can change your user name instead.")
• the link in /app/views/preferences/index.html.erb

[sarken]

Just a couple quick frontend suggestions since we were talking about text changes!