fix: Add custom domains to CORS allowlist

[DafyddLlyr]

What does this PR do?

• Fixes a CORS issue when requests go from custom domains to api.editor.planx.uk
• This was introduced here - https://github.com/theopensystemslab/planx-new/pull/2634/files#diff-a23b8e0cdc75fc858dc3a73983f67d257f78158596c37832202cf963efcf0a6aL42

How is this resolved?

• Adding a new env var CORS_ALLOWLIST
• This is only populated on production where the API needs to be accessed from both editor.planx.uk and planningservices.COUNCIL.gov.uk
• Rather than adding a new place to update the custom domains list I'm reading from the existing variable in the infrastructure project
• In non-prod environments, we can just call back to the current value which is the EDITOR_URL_EXT env var

To test

• I can log in / log out locally ✅
• I can make a non-auth API request locally (e.g upload file) ✅
• I can log in / log out on Pizza ✅
• I can make a non-auth API request on Pizza (e.g upload file) ✅
• I can log in / log out on Staging ✅
• I can make a non-auth API request on Staging (e.g upload file) ✅
• I can log in / log out on Prod ✅
• I can make a non-auth API request on Prod (e.g upload file) ✅
• I can log in / log out on a custom domain ✅
• I can make a non-auth API request on a custom domain (e.g upload file) ✅

[github-actions]

Removed vultr server and associated DNS entries

[DafyddLlyr]

As this is just a "derived" env var, we actually don't need to add it to the .env files and we can just populate it here.

If this is unclear / unhelpful I can add it to the .env and .env.example files.

[jessicamcinchak]

Pizza working as expected for me, thanks for getting to the bottom of this one!