Extract database gateways

Closes issues0#344

What does this MR do?

Defines a new pattern for database access:

• uses PDO (instead of mysqli)
• uses composition-over-inheritance to only include relevent methods in each gateway
• introduces dependency injection (so you can easily define which resources you need in a given controller, for example)

TODO:

• remove global PDO
• configure DI PDO from config.inc.*.php variables
• get DI PDO to work with the wrapped DebugBar PDO thing
• remove duplication of DI configuration between container test helper and setup.php
• check it a bit more....

How confident are you it won't break things if deployed?

Needs more checking...

Links to related issues

Checklist

• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
• joined #foodsharing-beta channel at https://slackin.yunity.org
• added an entry to CHANGELOG.md (description, merge request link, username(s))