Flesh out restricted field and group names

Split out from the coding-standards cleanup (#147), which removed commented-out stubs and their `// TODO: Build this out` notes so their intent could be tracked here.

`is_restricted_group()` is currently a stub that always returns `false`, and `is_restricted_field()` only checks a small hardcoded list (`post_title`, `post_author` for posts; empty for pages and users).

**Intended work**
- `is_restricted_group()`: block registering groups whose slugs collide with built-in metaboxes (e.g. title, custom-fields, revisions, author).
- `is_restricted_field()`: expand the restricted-field lists per object type as needed.

These checks are intended to feed the registration validation work in #153.

Start at the definitions of is_restricted_group() and is_restricted_field(), then review the built-in metabox slugs and existing restricted-field lists described in this issue. Confirm the expected object types and registration-validation usage in #153; done means group collisions are blocked and restricted fields are covered for the relevant object types.

Written by the indexing model from the issue text.