Move validation and authorization checks to call

[marten]

There's also https://github.com/Factlink/pavlov/pull/23 but that branch conflates the move with the addition of an authentication check, so we might want to merge this, and turn that into a request for the addition of the authentication check. That branch needs some documentation on why that addition is needed and how you would use it.

[coveralls]

Coverage remained the same when pulling b2b67b412f7548ef179f763cab6e0d0eb885eeb6 on feature/move-checks-to-call into 4df36e28ca880d826e5333bcf01beff990136940 on feature/new-api-proposal.

[coveralls]

Coverage decreased (-0%) when pulling b2b67b412f7548ef179f763cab6e0d0eb885eeb6 on feature/move-checks-to-call into 4df36e28ca880d826e5333bcf01beff990136940 on feature/new-api-proposal.