Expand CustomValue::_checkAccess()

Expand CustomValue::_checkAccess()

Before: Reports that access is available based one delegated-check for `Contact.update`

After: Reports that access is available based on multiple checks:

  1. The user must have access to the relevant CustomGroup (by way of ACL or perms)
  2. The user must have acces to the underlying entity (by way of checkAccessDelgated)

Comments: I did a bit of testing with `Custom_*.get`, and it does seem to
give access to single-value CustomGroups. So I removed a comment about
multi-value CustomGroups and expanded to a larger list of entities.