AO3-6672 Allow some admins to set a generic username

[brianjaustin]

Pull Request Checklist

• Have you read "How to write the perfect pull request"?
• Have you read the contributing guidelines?
• Have you added tests for any changed functionality?
• Have you added the Jira issue number
  as the first thing in your pull request title (e.g. AO3-1234 Fix thing)
• Do you have fewer than 5 pull requests already open? If not, please wait
  until they are reviewed and merged before creating new pull requests.

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!

[Bilka2]

Besides the comments I left, I think it would be good to have a feature test that goes through renaming a user as an admin. Just to make sure nothing gets missed because the specs test pretty close to the implementation itself.

[sarken]

I was reviewing in two tabs simultaneously so I could go back and forth between the views and the locales, so I'm sure this will be quality. If anything is confusing or contradictory, my bad, please yell!

[brianjaustin]

I needed to make changes here to fix issues with how @invitation is referenced (locals versus just passing it in). For everyone's sanity, I'll do the rest of the erb lint fixes in #5000 since that has most of them already anyways

[Bilka2]

Thank you!

[sarken]

(Sorry for all the comments! If it makes you feel any better, the review took me three days to finish. 😆)