AO3-6440 Configure admin and user reset token expiry separately

[sarken]

Issue

https://otwarchive.atlassian.net/browse/AO3-6440

Purpose

• Add a separate ArchiveConfig.DAYS_UNTIL_ADMIN_RESET_PASSWORD_LINK_EXPIRES variable for configuring how long admin password reset tokens are valid.
• Update both admin creation and admin password reset emails to mention the new value.
• Also update the admin creation email to say what to do if the set password link doesn't work.

Testing Instructions

• Create a new admin account for an email you have access to; ensure the email says the link is good for 5 days and the text is otherwise clear. Then delete said account.
• Request a password reset and ensure the email says the link is good for 5 days.
• Ideally wait 5 days to confirm the reset expires then and not in 7 days, but I think we're going to deploy before then, so we can double check on production.

Credit

Sarken, she/her