feat: ignore cors during connectivity check

[matthewhilton]

The issue
Regarding the connectivity check, its purpose is to detect if a url is accessible and if it is, redirect to the idp login page.

I found through some testing that the CORS policy enforcement was likely causing lots of false negatives i.e. saying it couldn't connect when the user actually could.

The fix
We can use fetch() with no-cors mode, this returns an opaque response i.e. no http code, body, etc.. it only returns yes or no did it connect which is exactly what we need.

See https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API/Using_Fetch#making_cross-origin_requests for info re no-cors

Testing
Manual testing - accessible site resolves the fetch promise, inaccessible site returns error where previously a accessible site would return an error because of cors

[Peterburnett]

Approved once CI is green

[Peterburnett]

#816 (comment)

Matt has noted here that the CI issue is a long term issue, and I've confirmed PHPUnit is clean. This is as good as it can be without tackling the behat fails directly. Approving and merging.