Expand list of specified role names and specify associated permissions

Problem/Motivation

Currently the Kit spec lists three roles: anonymous user, authenticated user, and administrator. While this is a good start, it doesn't provide sufficient guidance to prevent a proliferation of overlapping generic use role names. See #1738364: Add guidelines for handling of generic or cross-feature components.

Proposed resolution

Add the following list of roles:

### Generic user roles
As well as the required user roles, the following generic role names should be used wherever applicable:

- `contributor` is a Drupal user who contributes content, for example, a staff member at an organization or company.
- `editor` is a Drupal user responsible for editing and administrating content, taxonomies, and comments.

A feature may provide additional roles, but such roles should have a scope no greater than the scope of the feature. For example, a blog feature might provide a `blogger` role. However, a technology blog feature should not provide a `blogger` role, since that role would be relevant to a more general feature. Any new role that has broad, cross-feature or cross-distro relevance should be considered first for addition to the list of generic roles specified here.

### Permissions
The editor role should not be expected to have `administer nodes` permissions.

For each content type introduced, the editor role should be assigned the following permissions:

- `create [type] content`, `delete all [type] content`, `edit all
[type] content`

where [type] is the machine name of the content type being introduced by the
feature.

If the content type is appropriate for a site contributor to post, the contributor role should be assigned the following permissions:

- `create [type] content`, `delete own [type] content`, `edit
own [type] content`