Use Namespace policy class as base class for all derived namespace policies

As stated in namespace_policy.rb

  # NamespacePolicy has been traditionally for user namespaces.
  # So these policies have been moved into Namespaces::UserNamespacePolicy.
  # Once the user namespace conversion is complete, we can look at
  # either removing this file or locating common namespace policy items
  # here.

NamespacePolicy should no longer continue to inherit from ::Namespaces::UserNamespacePolicy and will instead become the base for common Namespace policy logic as described in &6689 (comment 715514674).

As mentioned in &6689 (comment 715767909):

app/policies/namespace_policy.rb sounds like a good candidate for the base policy shared by user/group/project namespaces