Model.hs : Add TypeSynonymInstances which is required for persistent 2.0...

..8.

Without this I get:

    Model.hs:13:1:
        Illegal instance declaration for
          ‘ToBackendKey
             persistent-2.0.8:Database.Persist.Sql.Types.SqlBackend Email’
          (All instance types must be of the form (T t1 ... tn)
           where T is not a synonym.
           Use TypeSynonymInstances if you want to disable this.)
        In the instance declaration for
          ‘ToBackendKey persistent-2.0.8:Database.Persist.Sql.Types.SqlBackend Email’

    Model.hs:13:1:
        Illegal instance declaration for
           ‘ToBackendKey
             persistent-2.0.8:Database.Persist.Sql.Types.SqlBackend User’
          (All instance types must be of the form (T t1 ... tn)
           where T is not a synonym.
           Use TypeSynonymInstances if you want to disable this.)
        In the instance declaration for
          ‘ToBackendKey persistent-2.0.8:Database.Persist.Sql.Types.SqlBackend User’

I would have commited it directly, but wasn't 100% sure this was right.