Add Diffusion policy capability "Can Edit and View Identities"
Summary:
Make it possible not to allow anyone to edit Diffusion identities.
Make it possible not to allow anyone to view other users' email addresses.
Closes T15443
Test Plan:
* As an admin, go to `/applications/view/PhabricatorDiffusionApplication/` and see new policy "Can Edit and View Identities" set to "All Users" (as implicitly before)
* As an admin, go to `/applications/view/PhabricatorDiffusionApplication/` and change "Can Edit and View Identities" from "All Users" to "Administrators"
* As a non-admin, go to `/diffusion/identity/` and try to select the disabled "Create Identity" button; get an error message clicking it due to lack of permissions
* Given there is at least one identity defined, as a non-admin, go directly to `/diffusion/identity/view/1/` and get "You do not have permission to view this object."
* Given there is at least one identity defined, as a non-admin, go directly to `/diffusion/identity/edit/1/` and get "You do not have permission to view this object."
* As a non-admin, go directly to `/diffusion/identity/edit/form/default/` and get "You do not have permission to edit this object."
* As a non-admin, go directly to `/diffusion/identity/` and get "No Identities found." instead of seeing the existing identities listed.
* As an admin, go to `/diffusion/identity/` and still see the existing identities listed.
* As an admin, go to `/diffusion/identity/`, select "Create Identity" to go to `/diffusion/identity/edit/` and see the "Create Identity" page (though broken; see T15453)
* As an admin, go to `/diffusion/identity/view/1/` and still see the existing identity.
* As an admin, go to `/diffusion/identity/edit/1/` and successfully edit the existing identity.
Reviewers: O1 Blessed Committers, speck, valerio.bozzolan
Reviewed By: O1 Blessed Committers, speck, valerio.bozzolan
Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T15443
Differential Revision: https://we.phorge.it/D25450