epic: Fix Event model `must_attend`, `should_attend`, `could_attend` fields

### Overview

We need to fix issues relating to the `must_attend`, `should_attend`, `could_attend` fields of the `Event` model so that it's decently usable. One is they should accept null inputs, making them optional if needed. Another is that they should probably validate the JSON input.

### Action Items

- [ ] issue to allow null for `must_attend`, `should_attend`, `could_attend` fields
- [ ] issue to validate JSON inputs to these fields for `practice_area` and `permission_type`
- [ ] issue to create admin widget for selecting the fields from possible values

### Resources/Instructions
- Spreadsheet: [PD: Table and field explanations, Current Field Permission tab](https://docs.google.com/spreadsheets/d/1x_zZ8JLS2hO-zG0jUocOJmX16jh-DF5dccrd_OEGNZ0/edit?gid=749942651)
- This issue came out of another issue: https://github.com/hackforla/peopledepot/issues/554#issuecomment-3363540665

> - I found a problem where I can't create events with empty lists in any of `must_attend`, `should_attend`, `could_attend` fields. They need to be made into null=True, blank=True in the model to accept `[]` in the JSON input. I think we do want them to be nullable.
>
> - Another thing is these fields are difficult to fill in with the JSON data they expect. It should be possible to make the admin UI widget load the `practice_area` and `permission_type` values for the user to choose from.