Centralize configuration in a config file

[j6k4m8]

we should have a single js config file that includes the root URL for the API (among other global config things) which open-source forks might want to change. our default should be https://api.biascorrect.org, but this should be easily swappable in a single place in the repo.

Plan (14 points)

• Find/read/evaluate/do a tutorial on how to do this (2 dev * 3 points = 6 points)
• Make a plan with @j6k4m8 or @ProfJanetDavis (2 dev * 1 point = 2 points)
• Follow the plan (2 dev * 3 points = 6 points)

[j6k4m8]

I'd recommend at least the following:

{
    "API_BASE_URL":            "https://api.biascorrect.org",
    "GOOGLE_ANALYTICS_ID":     "ga-####"
}

in something like config.production.json, where we might also have something like config.test.json with http://localhost:5000 instead.

See discussion at #43 for details.