Add the special page resource base class

Problem/Motivation

Every site has a handful of pages the front end addresses by role rather than by id — the home page, the blog index, the contacts page. Today the ids are constants in the code of each fork, which is the clearest single example of why the module cannot be installed elsewhere.

Proposed resolution

• MyrestSpecialPageResourceBase resolving the entity through the special_pages convention by role name.
• A role that is not filled answers 404 with the module's error envelope and a message naming the role, rather than a 500 or an empty 200.
• Environment-specific ids supported through a settings.php override, since dev and production hold different node ids.

Remaining tasks

Everything.