Allow for configurable Google Analytics key

[jmgasper]

We added GA to the check-email page here:

eb4cbb5

We have recently found out that dev and prod use different keys (GTM-MXXQHG8), so we want to make sure that the page knows to use the correct key in the correct domain (Topcoder.com vs Topcoder-dev.com)

• Prod: GTM-MXXQHG8
• Dev: GTM-W7B537Z

[suppermancool]

@jmgasper done in
tc-auth-lib-350.patch (this patch against commit eb4cbb5 in dev branch)

the GA_KEY key should be set inside the environment of the https://accounts-auth0.topcoder-dev.com project. Please set GA_KEY=GTM-MXXQHG8 in the prod environment, and GA_KEY=GTM-W7B537Z in the dev environment. GA_KEY should be in the environment next to DOMAIN, AUTH0DOMAIN, DICE_AUTH, ...