add Hets::Options including access_token
Introduce and use AccessToken model
add specs for access tokens
remove keepalive functionality
Ensure creation of fresh token.

Although unlikely, it can happen that an already
existing token is being generated.

This change ensures the generation of a fresh token.
Add unexpired scope.
Pluralize name of has_many relation.
Check access token for Repository :show.
Add access token hets call spec.
Merge branch '558-access_token_for_private_repositories' into develop