Code coach modularize

[ascholerChemeketa]

• Moved Pyflakes code coach to its own file.
• Update coach run/render code to more cleanly support multiple coaches.
• Set up Code Coach string for internationalization.
• Fix bug where coach div not removed if all issues fixed

Coaches are added to a list during ActiveCode construction.

runCoaches function is triggered on program run. It asynchronously runs all coaches, but then waits for all results so Code Coach div can display any coach feedback deterministically in the order coaches were created.

Questions/Comments:

• I could definitely see a case for more flexible addition of coaches to ActiveCodes instead of hard coding based on language in the constructor. I just don't have a clear idea of what that should look like.
• I think the internationalization is right...

[ascholerChemeketa]

I stress-tested the deterministic output with a TestCoach that just waited a random amount of time before returning a set string. Works as intended when there are multiple coaches taking variable amounts of time.

[ascholerChemeketa]

One aspect that might be annoying in existing books - Pyflakes complains about * imports.

Personally, I am using this as an excuse to replace them all in my book. But it would not be too hard to filter them out in the Pyflakes coach.

[ascholerChemeketa]

Updated to filter import * related warnings from pyflakes to avoid scary warnings in existing books.