Add default introductory code example

Instead of having logic for a fallback in the backend, we're choosing a
default for the snippet file.

For tracks that have core exercises, we pick the first core exercise.
For tracks without these, we pick the first exercise listed in the config.

Implement suggestions from #410

Move README includes

Fixes #718

Per
https://github.com/exercism/docs/blob/ccdf58f97f698c265f0f67fa7cb9d94084d7f0a3/language-tracks/exercises/README.md#files

change: update to v1.1.0 tests

Move readme insert

Fixes #720 replace lambda with method ref

This reverts commit 43e792bf55f3e53ae696b8480ba8fe73bac5bba0.

Remove unused images

Fixes #716 remove redundant type params

Fixes #696

Fixes #737

Use string builder over looped string concatenation.

ABOUT.md: remove confusing sentence

ABOUT.md: update formatting for Nextercism

[WIP] Adds additional test for two-fer exercise

Choose core exercises

house: add to track