Replace all calls to node entity methods with service equivalents and/or Typed Entities

[Ambient-Impact]

See #4 for more information.

• Node::getWikiNodeType(): this is a static method that just returns a constant; can be moved to a public constant on a final interface as a single source of truth.
• Node::getWikiNodeDateFieldName(): this is a static method that just returns a constant; can be moved to a public constant on a final interface as a single source of truth.
• Node::isWikiNode(): all uses of this can be pointed to the wiki node resolver service; note that WikiNodeResolver::isWikiNode() calls the node method of the same name, so that will have to be changed to either directly check the node's content type or re-implemented as a typed entity method.
• Node::getWikiNodeDate(): this can be implemented as a typed entity method.
• Node::getWikiNodeRevisions(): just a wiki node revision method wrapper; replaces all uses with service method.
• Node::getWikiNodeRevision(): just a wiki node revision method wrapper; replaces all uses with service method.
• Node::hasPreviousWikiNodeRevision(): see next method:
• Node::getPreviousWikiNodeRevision(): this contains a lot of logic that should be moved to the wiki node revision service or a typed entity class.
• Node::isMainPage(): this is just a wrapper around the main page service method; replace uses with service call.
• Node::isHiddenFromSearch(): this mixes main page logic and the hidden from search field; rework so this module doesn't need to know about or use such a method; see Move the field_hide_from_search configuration to the omnipedia_search module if possible #2.
• Node::addRecentlyViewedWikiNode(): this is just a wrapper around the wiki node viewed service; replace single instance with a call directly to the service.