Improvements to automatic refreshing of OAuth access tokens

[floehopper]

@grega @patch0 @fspeirs If they work as intended in experience-cs, it might be worth extracting some of the changes in https://github.com/RaspberryPiFoundation/experience-cs/pull/1425 into this gem:

• https://github.com/RaspberryPiFoundation/experience-cs/pull/1425/commits/759c8450983ed6e23c4bf949570ca82d5d6896a6
• https://github.com/RaspberryPiFoundation/experience-cs/pull/1425/commits/7d523151924640d79f20bed39deb0cf6ffb8331a