Pull/reimplement progress bar from Turbo so that it can be used by other implementations and so we have more control over it

Problem/Motivation

The progress bar that Turbo provides is decent but it doesn't expose much in the way to customize the HTML or better hook into its lifecycle. We can adapt it or a similar third-party component to have full control over it and improve it as we need. This will also help in styling/debugging it, as it's a bit annoying to do currently by finding a place to set a breakpoint in the dev tools debugger.

• turbo/src/core/drive/progress_bar.js
• rstacruz/nprogress: For slim progress bars like on YouTube, Medium, etc - the code is fairly dated but a good inspiration, especially the demo/debug page

Steps to reproduce

See above.

Proposed resolution

Write or adapt our own version, and disable Turbo's progress bar.

Remaining tasks

TBD.

User interface changes

Minimal - progress bar should be about the same by default.

API changes

Mostly adding ways to customize or hook into the progress bar.

Data model changes

None.