Remove the partial guideline

* https://github.com/thoughtbot/guides/commit/a2428be0f85fa30c0c30dba8d5843e86ff1163f8#commitcomment-8468894
Use `ENV.fetch` instead of `ENV[]`

It's easy to forget to set environment variables (e.g. `AWS_SECRET_KEY`) on
staging or production. Using `ENV.fetch("AWS_SECRET_KEY")` ensures that missing
environment variables raise errors instead of just being `nil`.

Heroku boots the app on deploy, which runs the app's initializers. Often,
environment variables are set in an initializer. This combination means that if
an environment variable isn't set, an error will be raised and Heroku will abort
the deploy. This means users can never see a version of the app without the environment
variables set.
Add Angular Best Practices

A first crack at some Angular best practices arrived at through a seven
month angular project and discussions with other teams doing angular
work around the company. I'm no angular expert, but these are practices
I can remember working well, particularly in the face of other options
that *did not* work equally well.
Prefer `reduce` over `inject`

`reduce` provides symmetry with our already-preferred `map`.
Additionally, it has a more intention-revealing name and is more
approachable for people unfamiliar with it. "I am reducing this array to
something". `inject` doesn't make the same sense.
Don't include header comments in swift
Only follow a best practice if you understand it

The styles should always be followed, and anything that is a style
guideline should be defensible with just "we picked this style."

The best practices are things that we've learned through experience and
teaching, and therefore cannot be conveyed completely in a pithy
guideline. They are reminders for those who understand them, not lessons
for those looking to learn.

As such, if you do not understand a best practice guideline, then you
are simply not the target audience.