Feature: Add default account overrides with .hsaccount file

[kemmerle]

Description and Context

In this PR, I've implemented support for default account overrides with the new centralized configuration file.

Now, customers can place a .hs-account file in any directory where they would like the default account overridden. The .hs-account file will contain a single numerical entry: 1234567, or the account ID. Any changes made in the directory and its subdirectories will apply in the account stipulated in the .hs-account file. This will work very similarly to our nest configs currently do.

The advantages are:

1. We will still have one centralized configuration file and therefore one single source of truth.
2. We will create a command to auto-generate the .hs-account file, so that customers need never have to manually interact with the config.
3. It's much more secure, if a customer accidentally uploads the file to GitHub--it contains no sensitive information besides an account ID.

TODO

• Address feedback

Who to Notify

@brandenrodgers @camden11 @joe-yeager

[brandenrodgers]

The code in here seems reasonable to me! I'll test this out early next week 🎉

[brandenrodgers]

Just one suggestion to add another try/catch, but otherwise this all lgtm!