Adaption validate.py to relational database design

[r-peschke]

Start on branch feature/relational_db
The validation-script should pass the tests with the current models.yml. There are some new attributes like reference, deferred and unique. I'm sorry, but there will be more to check.
There is a make target defined as validate-models.
There is a debugger activated, to work you should copy the following code into youtrlaunch.json (VS-Code, Codium):
{
"name": "Python: attach META 5678",
"type": "debugpy",
"request": "attach",
"connect": {
"host": "localhost",
"port": 5678
},
"justMyCode": false,
"pathMappings": [
{
"localRoot": "${workspaceFolder}/openslides-backend/global/meta",
"remoteRoot": "/app"
},
]
},

[luisa-beerboom]

@r-peschke While I am thankful for the explanation on how to set up my work environment in the meta, the description leaves me guessing on what the adaptation would actually entail.

I will try to discern what needs to be done from the code, but if you find the time, could you please at least give me some pointers on what I will have to pay attention to? That would probably greatly speed up the work.

[jsangmeister]

Also, this needs to be done in #17 or, if wanted, in a common feature branch after #17 is merged

[jsangmeister]

In #61, the model validation was disabled in the feature/relational-db branch. This should be reverted as part of this issue.

[jsangmeister]

In #84, the validation is rudimentary enhanced so that the current models.yml passes. This way, the CI can be re-enabled. The validation for the new fields should be enhanced to check what fields are allowed in which cases and to further validate the content of the fields.