Use DATABASE_URL for MySQL

[halilim]

Description

• Currently the database config uses hardcoded values for Forge instead of making use of DATABASE_URL environment variable.

Solution
Use $host, $database, $username and $password variables in MySQL config, similar to PostgreSQL; and default to existing Forge values in case of absence. Additionally, a $port variable can be extracted from $databaseUrl and used similarly.

Additional context
I am trying to install Firefly III on a Dokku instance. Dokku describes itself as a "mini-Heroku". It makes use of DATABASE_URL in the same way. Thus, I think this change would be useful for Heroku MySQL/MariaDB add-ons and Dokku MySQL/MariaDB plugins.

I can create a pull request. I just wanted to discuss this first per the contribution guideline. And I noticed there is a sqlsrv config too, which should be able to function the same way.

[JC5]

That's a good idea. I have some changes lined up for those files, for the upgrade to 4.8.0. Could you hold this until 4.8.0 is released?

[halilim]

Sure thing, waiting for 4.8.0 then (& using dokku config:set FF_DB_... in the meantime).

[JC5]

https://github.com/firefly-iii/firefly-iii/projects/5#card-20337815