Add ReadAdmin and WriteAdmin grants

[elv-nate]

No description provided.

[elv-gilles]

I am not too hot with that change (but not opposed):

Originally we had:

	Create    Grant // allow creation of an item
	Access    Grant // allow to know that an item exist, e.g. in a 'list'
	Read      Grant // allow to read an item
	Update    Grant // allow to modify an item

where each of these 'grants' are used on specific sub-interfaces of the auth context:

type AuthContext interface {
	Space() Space
	Lib() Lib
	Q() Q
...

Then, following some emergency, we added:

	ReadCrypt Grant // read from content crypt

whereas we instead should have been using a Read grant on a Crypt interface of the auth context.

So - same here - we should have an 'Admin' interface on the auth context and use the existing grants.