Abstract Tooltip HTML (and any other HTML) into external template files

We should abstract Tooltip and any other HTML into external templates which can be referenced from the source. A global config param should be created which allows developers to point to different templates (e.g. their own HTML).

Start by locating the Tooltip HTML generation in the JavaScript source and inventorying other inline HTML. Define the global template configuration and verify that custom templates can replace the defaults without changing chart behavior; the issue provides no file or test names, so the surrounding configuration and test setup must be mapped first.

Written by the indexing model from the issue text.