Introduce Permissions Service

This PR is based on our discussion with @nickstenning.
It introduces the `permissions` angular service as a focal point of setting the permissions for an annotator and introduces some helper functions handling then,

I went a bit farther and realized that we're not really using the Annotator.Permissions plugin anymore, so this PR gets rid that plugin too.

What we can also do that we can rewrite some of the short function of the annotation directive to also use the permissions service.
I'm thinking of: 
https://github.com/hypothesis/h/blob/master/h/static/scripts/directives/annotation.coffee#L17
https://github.com/hypothesis/h/blob/master/h/static/scripts/directives/annotation.coffee#L82


`permissions.public` and `permissions.private` return permissions properties. `permissions.isPublic` doesn't read anything outside of the permissions property, so it should probably just take the permissions property: `permissions.isPublic model.permissions`.