Add ESLint rule to prevent usage of process.env.foo about blurts-server

[pdehaan]

Ref: no-process-env

Apart from /app-constants.js, we seem to use process.env.* in two other files:

1. blurts-server/email-utils.js

   Line 18 in eb90d10

   if (process.env.DEBUG_DUMMY_SMTP) {

2. blurts-server/routes/user.js

   Line 64 in eb90d10

   if (process.env.DEBUG_ALLOW_USER_LIST) {

[nhnt11]

Yeah, I made those specifically use process.env for now since they are debug flags and should not be mandatory to include in .env. We can probably modify app-constants.js to better handle debug flags.