Adds .gitattributes
Converts line endings to LF
Adds diff attributes to some file extensions

This will make `git diff` hunk-headers be better tailored to the
language grammar being diffed, and will make `git diff --word-diff` use
the correct word-diff-regex for the language grammar.

See "Defining a custom hunk-header" and "Customizing word diff" in
gitattributes(1) (`man gitattributes`).
Replaces @lists.learningu aliases with @learningu

Partially resolves Github issue #245. This will not be fully resolved
until the local_settings.py files for each chapter are updated as well.
Updates code_relicensing.py

Replaces @lists.learningu aliases with @learningu in the license
template for the useful script that relicenses all of the code.

Partially resolves Github issue #245.
Executes code_relicensing.py to relicense code

The main update is to replace @lists.learningu aliases with @learningu
in the licenses.

Partially resolves Github issue #245.
Merge pull request #1323 from learning-unlimited/gitattributes-and-lists.learningu

Merge branch 'gitattributes-and-lists.learningu' into main

Adds gitattributes and removes references to lists.learningu