bugfix: fix token expiry check in admin settings

[matthewhilton]

Issue:
There were a few issues found after more thorough testing of #636:

1. The class wasn't included (oops!)
2. There is a core bug which means on newer php versions if it didn't have an action link it would throw an exception -> https://tracker.moodle.org/browse/MDL-83537
3. The admin_setting_check doesn't work unless it is in the top level settings page and not in the client settings definition area, because of the way internally it searches through the admin tree to find the check.

Fixes:

1. Moved token expiry check to top level. Realistically all client sdks could/should implement this so no point having it only in the Azure client section.
2. Added action link as an fix until MDL-83537 is implemented
3. Fixed a spelling error

Testing/screenshots:
Where token expiry is implemented - in Azure client code:

Where token expiry is not implemented:

[rhell4]

Looks good to me.