Unify AccessToken and RefreshToken

[sunsingerus]

SUPPOSITION
App\Model\AccessToken and App\Model\RefreshToken are very much the same - the main difference is inside createToken()

PROPOSITION*
Let's extract common functionality into new App\Model\Token class and rework RefreshToken and AccessToken to be children of Token

PS. I can implement this

[sunsingerus]

ok, let's return later to tokens, it is not first-priority issue at the moment