Use updated validation API

[nicksellen]

Fixes #2614

What does this PR do?

A user struggled to sign up because they couldn't see the username had validation errors. I noticed that quite a few places were using the old vuelidate API to check for errors, so I fixed them all.

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 #karrot:matrix.org
• [ ] added an entry to CHANGELOG.md (description, pull request link, username(s))
• [ ] tried out on a mobile device (does everything work as expected on a smaller screen?)