Port templates

[MSP-9819](https://jira.tor.rapid7.com/secure/RapidBoard.jspa?rapidView=84&selectedIssue=MSP-9819)

Port templates for injecting ERDs into YARD docs on namespace Modules and ActiveRecord::Base subclasses into this plugin.

Additionally, show ERD for entire project on _index.html to handle Rails application, since they won't usually have a top-level namespace Module that covers all ActiveRecord::Base subclasses.