API change for how Metatag identifies compatible entities

Prior to v7.x-1.5, in order to enable Metatag for an entity type it was necessary to modify hook_entity_info() in order to add a 'metatags' attribute with a value of "TRUE" to either the main entity or one of its bundles. It was then also possible, via the Advanced Settings page (admin/config/search/metatags/settings), to disable support for some of these entities and bundles, should a site builder / administrator decide that meta tags were not wanted for a specific entity. Lastly, should there not happen to be a default configuration defined (admin/config/search/metatags) for that entity then the meta tag fields still would not be displayed on the entity's edit form for that entity type.

This structure led to scenarios whereby an entity was enabled via the hooks but would still not have options available from their edit forms because there was no default configuration defined or it had been disabled. Additionally it placed a burden on the module creator to decide whether Metatag supported was available for their custom entities, rather than leaving it up to the individual site builder.

The new system streamlines this process in three ways:

1. It is no longer necessary to modify the entity's definition in hook_entity_info, any entity that has view modes and is not a configuration entity may be supported automatically.
2. The site builder has full control over which entities can have meta tags added to them via the (renamed) settings page (admin/config/search/metatags/settings).
3. The presence of a default configuration (admin/config/search/metatags) no longer plays any part in deciding whether an individual entity may have its meta tags customized, it only controls the default values used for that entity/bundle.

It is hoped these changes will help simplify the lives of module builders and put full control back into the hands of each site builder.