Refactor hostname helper function

[jdeanwallace]

Related https://github.com/tiny-pilot/tinypilot-pro/issues/1162 Resolves https://github.com/tiny-pilot/tinypilot/issues/771

This PR refactors the futureLocation function to app/static/js/hostname.js and renames it to determineFutureOrigin. This change is needed to allow the function to be reused by https://github.com/tiny-pilot/tinypilot-pro/pull/1169

This is mostly a non-functional change with the following caveats:

1. Name change
   • Before futureLocation
   • After determineFutureOrigin
2. Docstring change
   • Before https://github.com/tiny-pilot/tinypilot/blob/34161c3c450f5bc3c99cffbd3cd0a778aaeed6dd/app/templates/custom-elements/change-hostname-dialog.html#L100-L107
   • After https://github.com/tiny-pilot/tinypilot/blob/d889c24044a510a5ba91464ac229dcd3c08bd578/app/static/js/hostname.js#L1-L8
3. Added guard on oldHostname variable
   • Before https://github.com/tiny-pilot/tinypilot/blob/34161c3c450f5bc3c99cffbd3cd0a778aaeed6dd/app/templates/custom-elements/change-hostname-dialog.html#L111
   • After https://github.com/tiny-pilot/tinypilot/blob/d889c24044a510a5ba91464ac229dcd3c08bd578/app/static/js/hostname.js#L16
4. Added tests
   • app/static/js/hostname.test.js

[jdeanwallace]

Automated comment from CodeApprove ➜

⏳ @jotaen4tinypilot please review this Pull Request