Development: allow changing mode to dev/prod without having to edit a code file

[gabrc52]

In order to switch from production to development, you have to switch CURRENT_MODE to "dev" in server_configs.py, and it may accidentally be staged and committed.

Additionally, there are scripts to run on production and testing, run_prod.sh and run_testing.sh but they don't change the mode, and it's a lot of steps to have to both run a different script and also change a line of code.

Using environment variables might be one possible solution, where the CURRENT_MODE is an environment variable and it is set at the shell scripts to run, or default to dev if the variable is not set.