Keen Modding Live - Better login experience

Today I rearranged the authentication part of Keen Modding Live a bit. Now the Keen Modding authenticator indicates whether an account needs to be linked or the logon just failed. The login page uses that information to only show the Link Accounts button if makes sense to even try. This adjusted API will make for a convenient login experience in Abiathar Live Studio as well, which is my next thing to implement.