Allow setting a single DB url environment variable

[savager]

Summary

Perhaps I'm doing something else wrong, but I thought I'd spin up a test instance of Shlink to evaluate, and having to set many environment variables instead of simply a typical postgres url seems unnecessary.

Especially in my circumstance where I'm using Google Cloud SQL Proxy's url formatting:

<DB_TYPE>://:@/dbname?host=/cloudsql/<PROJECT_ID>::<INSTANCE_NAME>

I've tried putting dbname?host=/cloudsql/<PROJECT_ID>::<INSTANCE_NAME> as the DB_NAME environment variable, but am still getting an error.

Initializing database if needed...
[ERROR] Error generating database.. Run with -vvv to see specific error info.

Use case

Setting a database url as one environment variable is easier, and cleaner.