Support turbo (and drop support for turbolinks classic, keep support for turbolinks5)

[timdiggins]

We have code that works against Turbolinks5 vs Tubolinks 4 ("Classic") vs no-turbolinks.
e.g. on_page_load.es6

With the release of Turbo (Turbo Drive, is the relevant component), I think we should be supporting and testing against Turbo as well as the last release of legacy Turbolinks (i.e Turbolinks 5.2.0 or 5.3.0-beta.1)

Suggested grid:

• 6.0 x Turbolinks
• 6.1 x Turbolinks
• 7.0 x both Turbolinks and Turbo Drive
• main x Turbo Drive only

(what about no turbolinks/turbo at all - we probably need to test this if we're supporting it, though we're not testing it at present).

Maybe we should consider duplicating the dummy app and make it dependent on Turbo::Drive but also making it use the rail 7 webpack-less approach to javascript. (in the same way as we have been supporting separately testing sprockets and webpack).