[web] Compute existingRole instead of existingRoleID in RoleActionsMenu

Summary:
This should be a noop refactor that I'm separating into separate diff to make next diff easier to remove.

We basically grab the RoleInfo instead of the RoleInfo.id so that we can access the specialRole field and deprecate useRoleDeletableAndEditableStatus.

Depends on D11205

Test Plan: flow, the invariant, logging the value, making sure RoleActionsMenu continues to look/work as expected

Reviewers: ginsu, tomek, kamil, michal

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D11206