Multiple portfolio flags bug: no check when editing an existing portfolio

[zandercymatics]

Current Behavior

Currently, you cannot add more than one userportfoliopermission per user when the "multiple_portfolios" waffle flag is off - which is expected. However, if you take an existing userportfoliopermission record and add the same user that way, the form lets you do this.

Expected Behavior

The multiple_portfolios waffle flag is intended to only allow one UserPortfolioPermission record per user.
The current check we have on new records should remain, but we should also deny the user from associating themselves with multiple portfolios by just editing another unrelated UserPortfolioPermission record.

Steps to Reproduce

1. Create two different UserPortfolioPermission records (with different portfolios and users)
2. On one of the records, set yourself as the user.
3. On the record that isn't set as your user, edit it.
4. Add yourself as the user to that record.
5. Note that now, technically, you are assigned multiple portfolios

Environment

No response

Additional Context

We're currently doing a form-level check for this, and that should be sufficient for this ticket as well.

Note: this may be related to the self.pk check in user_portfolio_permission.py.

Issue Links

• relates to by Design review: Analyst Portfolio view #2402