Follow-up from !18530: Consider renaming User#full_private_access?

The :full_private_access ability was renamed to :read_all_resources as part of !18530 (merged), but the method User#full_private_access? instance method was left to keep the size of the MR manageable. Consider refactoring it.

The following discussion from !18530 (merged) should be addressed:

• @reprazent started a discussion: (+3 comments)

  Now that this is an ability, perhaps we should take the opportunity to rephrase.

  Now it means "read all resources", which is allowed for admins and auditors (those are the conditions we check). For admin we also enable writing for most resources, but we never do for auditors. So I think we should try to have the ability describe what it's enabling/disabling.

  So perhaps rephrasing the ability to read_all_resources makes more sense? But let's keep User#full_private_access as a method name.

  What do you think?

/cc @reprazent @ifarkas