Created CODE_GOV_CONFIG_JSON env variable

[DanielJDufour]

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Summary

This PR fixes/implements the following bugs/features

• Creates CODE_GOV_CONFIG_JSON environmental variable that points to the re-users site configuration JSON file
• Replaced " with ' per ESLint standard

Explain the motivation for making this change. What existing problem does the pull request solve?
This solves the problem that potential re-users would have to fork code-gov-front-end and manage syncing with the upstream repo. For example, if we changed a file in the main code-gov-front-end repo, that could overwrite their changes in their own version if they run git pull upstream master

Test plan (required)

Demonstrate the code is solid. Example: The exact commands you ran and their output, screenshots / videos if the pull request changes UI.

[froi]

Changes look good.

Checked out the branch and tested locally. I like the change to override the location of the site config file.