Add conditional config for the UX tier

[matt-bernhardt]

Why are these changes being introduced:

We have a long-term multidev for the UX team to use for content work, but it isn't consistently implemented between several names. This causes problems when trying to access the network dashboard and password reset interfaces, because the inconsistent names provoke a server error.

How does this address that need:

This adds a conditional to web/wp-config.php to standardize how the UX tier is named. This is the same conditional where we assign custom names for the Dev, Test, and Live tiers - so this is a consistent place to assign a custom name for a multidev.

Document any side effects to this change:

None that I can see - although this is the first time we're using this approach for a multidev, so that may be worth noting.

Relevant ticket(s):

https://mitlibraries.atlassian.net/browse/pw-92

Developer

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• ANDI or Wave has been run in accordance to
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  all issues introduced by these changes have been resolved or opened as new
  issues (link to those issues in the Pull Request details above)
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NO dependencies are updated

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