Remove isLoggedIn and use variant with user payload instead

[MargaretKrutikova]

According to the source code of react-netlify-identity isLoggedIn is derived from the user object (https://github.com/sw-yx/react-netlify-identity/blob/master/src/index.tsx#L353) which means it should be mapped to a variant in reason. Smth like this:

type t('a) =
  | Anonymous
  | LoggedInUser(NetlifyUser.t('a));