Php 7.4

What does this MR do?

• Use PHP 7.4 images
• Get rid of a HTTP client component used in mail tests that is incompatible with PHP7.4
• Update to latest symfony 4 version
• Fix some notices PHP7.4 introduces in our code. There might be some left, they are not triggered in production just during dev/test.

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

It doesn't work yet.

Links to related issues

Any relevant links (issues, documentation, slack discussions).

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))
• Once your MR has been merged, you are responsible to update the #foodsharing-beta Slack channel about what has been changed here. They will test your work in different browsers, roles or other settings