API: add endpoint for restricting files

[pdurbin]

As far as @landreev and I know the ability to restrict (or unrestrict) files is limited to the GUI so I'm creating this issue as a feature request for the API. Personally, I would use it in integration testing (for #2466 (comment) for example) but I imagine it would be useful for third party developers as well.

[pdurbin]

Not having this endpoint for restricting files means it's not possible to write an API-only integration test for #2648 . In theory, I could use Selenium to restrict a file through a browser, but that's a ton of work.

[pdurbin]

@raprasad and I discussed this the other day while we opened #3422. When we make a list of "GUI only" features, this issue should go on it.