Use session as an instance

What does this MR do?

Switched to accessing the session via instance methods (instead of static access).

I fixed a few other little things on the way through too.

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

Seems ok...

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))