Script and CI job to print outdated deps to slack

What does this MR do?

To keep dependencies up to date it's nice to know what is out of date.

This adds a script that will print the outdated dependencies to slack, more specifically will run:

• composer outdated -D
• yarn outdated (in each of client/ and chat/ directories)

You can check the #fs-outdated channel in slack now to see the output from my local testing :)

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

Should be fine!

Links to related issues

Checklist

• added a test, or explain why one is not needed/possible... ... just tested locally
• 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