Add support for enforcing YubiKey login depending on client
#609 added support for not using YubiKey login when certain client applications was used.
In some cases it might be desirable to enforce that YubiKey login is used (see #891). Typically, the normal behavior is to also accept users that has not been assigned a YubiKey (if they provide their regular username and a correct password).
When a client-application has been configured for enforcing YubiKey login it will not be possible for users without a YubiKey to login with that client.
The suggested fix is to specify a list of client application ids in the yubikey.properties file. Eg:
enforce-yubikey = net.sf.basedb.reggie.delivery