Follow-up from "WIP: Support group based restriction"

The following discussions from !14 (merged) should be addressed:

Would it not be sufficient to have
    category = SelectField("Category", coerce=str, validators=[DataRequired()])
here?

@ip914 started a discussion:
The above would make this code below redundant.
@ip914 started a discussion:

Is there any strong reason why we don't want to have the conventional id primary key? What does a code represent?
@ip914 started a discussion:

Why not having a relationship rather than a plain attribute? Referential integrity is not respected here but maybe that's OK?
@ip914 started a discussion:
```
        return bool(self.on_behalf)
```
@ip914 started a discussion:

What if there's no Person for the current username? You would get a RuntimeError
@ip914 started a discussion:

Same here
@ip914 started a discussion:

And here
@ip914 started a discussion:

And here too

    if not project or project.category not in category_codes:
        can_edit = False

    if not project or project.category not in category_codes:
        can_view = False