AO3-6560 Add Tumblr links to error pages

[scottsds]

Pull Request Checklist

• Have you read "How to write the perfect pull request"?
• Have you read the contributing guidelines?
• [N/A ] 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/AO3-6560

Purpose

Adds Tumblr links on various error pages so users have an additional non-twitter source of news about AO3.

Testing Instructions

Confirm the following pages have working links to Tumblr:

1. https://test.archiveofourown.org/502.html
2. https://test.archiveofourown.org/503.html
3. https://test.archiveofourown.org/507.html
4. https://test.archiveofourown.org/nomaintenance.html
5. https://test.archiveofourown.org/support

References

These changes were originally submitted here: #4575 by another contributor, who decided to close their PR.

Credit

What name and pronouns should we use to credit you in the Archive of Our Own's Release Notes?

he/him

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

[sarken]

Can you run the i18n tasks on this branch? That'll clear up the test failures.