[BUG] Validate owner in self-assignment API

[athiruma]

• QUADS version (rpm -qa | grep quads and/or quads --version):
• Python version:
• Operating System:

Describe the bug
A clear and concise description of what the bug is.
curl -k -X POST -H "Authorization: Bearer $TOKEN" -H "Content-Type: application/json" -d '{"description": "Short description here", "owner": "hello_quads", "qinq": 1, "vlan": "602"}' http://quads2-stage.example.com/api/v3/assignments/self

The owner must be a valid user, nice to have user validation.

To Reproduce
Steps to reproduce the behavior:

1. Go to '...'
2. Click on '....'
3. Scroll down to '....'
4. See error

Generate TOKEN for test
In payload: input owner with any random word.

Expected behavior
A clear and concise description of what you expected to happen.
The owner must be an active or valid user.

Screenshots
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Additional context
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