Support providing the API key through settings.php

I took inspration from 🐛 Overridden API on settings doesn't expand config form Closed: duplicate and improved the experience when the COI module is not installed. In that case, if the API key is overridden:

• it is entered in the text field, but the field is disabled so it cannot be changed
• because of this all other settings are visible and the field can stay required
• the API key is still being validated, I don't see why we would make that exception if the key comes from settings.php

I also took the README changes from 🐛 Overridden API on settings doesn't expand config form Closed: duplicate . I credited @spncr in this issue.

DieterHolvoet → credited spncr → .

DieterHolvoet → credited hanoii → .

Automatically closed - issue fixed for 2 weeks with no activity.