Add additional configuration variables

[codesections]

We should configuration options to match the configuration options in tootsuite/mastodon, as described in the documentation.

Specifically, we should add the following variables:

PostgreSQL

• [x] DB_HOST
• [x] DB_USER
• [x] DB_NAME
• [x] DB_PASS
• [x] DB_PORT
• [x] DATABASE_URL
• [x] DB_SSLMODE

Redis

• [x] REDIS_HOST
• [x] REDIS_PORT
• [x] REDIS_URL
• [x] REDIS_DB
• [x] REDIS_NAMESPACE
• [x] REDIS_PASSWORD

Deployment

• [x] PORT
• [x] SOCKET
• [x] BIND
• [x] RUST_ENV / NODE_ENV
• [x] RUST_LOG_LEVEL
• [ ] TRUSTED_PROXY_IP

Are there any others that we should also add?

[EDIT: We should also remove .env from version control (though we could have a example config). When doing so, it might also make sense to switch from pretty_env_logger to env_logger, which has essentially the same API and would let us set the default log level without an env variable.]

[Gargron]

• LOG_LEVEL
• SOCKET (for binding to Unix socket instead of TCP interface)
• BIND (for choosing TCP interface)
• DB_SSLMODE
• TRUSTED_PROXY_IP (which source IP to trust for forwarded IP info)
• REDIS_DB (in practice kinda useless because pubsub is global across all Redis databases)
• REDIS_NAMESPACE (very important)
• REDIS_PASSWORD

[codesections]

We also need to add an environmental variable that specifies what other environmental variables to use. flodgatt should read the other env vars from a .env file with that post-fix. For example, if the variable is set to development, flodgatt should read other variables from .env.production.

The user should be able to set this variable as NODE_ENV (for backwards compatibility) or as RUST_ENV (to be clearer). If both are set, RUST_ENV should take precedence.

[codesections]

Closing this for now. Depending on how Mastodon ends up handling TRUSTED_PROXY_IP, we may want to reopen later. Also, we currently support DB_SSLMODE only in that we acknowledge it and print a warning that we don't yet support using SSL with Postgres (#41). When we add SSL support, we'll obviously need to expand our handling of that environmental variable.