AO3-7384 Redirects to 404 page if user does not exist

[beckystar4]

Pull Request Checklist

• [ X] Have you read "How to write the perfect pull request"?
• [ X] Have you read the contributing guidelines?
• [ X] Have you added tests for any changed functionality?
• [ X] Have you added the Jira issue number
  as the first thing in your pull request title (e.g. AO3-1234 Fix thing)
• [X ] 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/AO3-7384

Purpose

What does this PR do? AO3-7384 Redirects to 404 page if user does not exist. This PR checks if the user is logged in, if a user is logged in but the user is nonexisting/null, it will redirect to 404.

Testing Instructions

How can the Archive's QA team verify that this is working as you intended?

**Log into AO3
Open the URL for the skins page of a nonexistent user, e.g. https://test.archiveofourown.org/users/deleted_user/skins
Verify it redirects to 404.

Log out of AO3
Open the URL for the skins page of a nonexistent user, e.g. https://test.archiveofourown.org/users/deleted_user/skins
Verify that it brings you to log in page.**

If you have a Jira account with access, please update or comment on the issue
with any new or missing testing instructions instead.

You can remove this section if there are already full testing instructions in the Jira issue.

Credit

What name and pronouns should we use to credit you in the Archive of Our Own's Release Notes?
Rebecca (she/her)

If you have a Jira account, please include the same name in the "Full name"
field on your Jira profile, so we can assign you the issues you're working on.
Rebecca (zipzipzip)

Please note that if you do not fill in this section, we will use your GitHub account name and
they/them pronouns.

[sarken]

Hi, Rebecca!

Thank you so much for this pull request. Someone will be along to review it soon.

In the meantime, I've updated the Jira issue status to In Review so no one mistakenly creates a duplicate pull request.

It sounds like you've already created a Jira account, but Jira has been unreliable about showing us new accounts in the admin panel lately. Can you reply here or send an email to otw-coders@transformativeworks.org with the email address associated with your Jira account? Once you've done that, we should be able to set up the permissions for you to comment on, assign, and transition issues.

Thanks again for contributing! If you have any questions, you can contact us at the same email address listed above.