Fixes #259 (closed).

This beefy MR explores an alternative design for access policy management in Gargantext which might supersede the existing withAccess and withAccessM.

It adds a type-level combinator called PolicyChecked, which can be used to embellish any servant route so that we can bring an AccessPolicyManager in scope which can be used to implement policy checking.

We have two new functions called withPolicy and withPolicyT, which both accepts a BoolExpr AccessCheck, which means we can compose policy checks using the similar boolean API mechanism. For example, the following grants access if the input NodeId matches the one of the logged in user, the authenticated user is the master user or if the node is a descendant of the authenticated user:

nodeChecks :: NodeId -> BoolExpr AccessCheck
nodeChecks nid =
  nodeUser nid `BOr` nodeSuper nid `BOr` nodeDescendant nid

I've also added tests to check that we are indeed protecting the endpoints as expected.

N.B. I haven't added this check for all the endpoint, that should follow in future MRs. However, I have left in place the old withAccess, which might be eventually superseded.