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.
Test hets api call (access-token, url-catalog).
Pass access token on redirect, fix IRI constraint

The IRI constraint fix is temporary. It will be removed as soon as the
final RouterConstraints are implemented.
Use sha hexdigest as token

The repository param, ontology-version path and current time will be
hashed via sha2 for the token.
Delete expired access tokens every day.
Obey Hound.
Line break after dot.
Move class methods to the top.
Put hash generation to a separate method.
Rename methods and variables and comment.
Don't break line after opening parenthesis.
Bring the `if` back.
Rename `options` to `hets_options`.