Create Table: CancelledEvent

### Dependency
- #151 will decide if we need this issue

### Overview

We need to create the CancelledEvent table so that we can store the reason and dates events were canceled, in order to analyze data and suggest future cancelations.

#### Details

A table and a model are the same thing

### Action Items

- [x] identify and document table description (see spreadsheet under Resources)
- [ ] if not, reach out to PD leads
- [x] compare and check off the data fields (below) against the ERD. Note any fields in the ERD that's missing from the list. (see Resources)
- [x] compare the check off associated tables (below) against the ERD. Note any associated tables in the ERD that's missing from the list (see Resources)
- [x] create a single model in Django (defining schema)
- [x] write a test for the relationships this model will have with other models (e.g., creating a user and assigning them a set of permissions on a project).
- [x] write an API end point
- [x] write API unit tests
- [x] document the endpoint
- Check to see if open issues in `FK status` below
- [x] If all Issues listed are closed (or there are no FKs), then skip the next 3 steps
- [ ] ~comment out the code after you create it (see FK Status below for open/closed state)~
- [ ] ~add an action item on the open issue to uncomment the code line you commented out when that issue's table is created.~
- [ ] ~Provide your file and line number as a permalink under resources in that issue~

### Resources/Instructions

- See [People Depot Resources wiki page](https://github.com/hackforla/peopledepot/wiki/Resources-and-Links) for links
- ERD
- Spreadsheet: [PD: Table and field explanations, Current Field Permission tab](https://docs.google.com/spreadsheets/d/1x_zZ8JLS2hO-zG0jUocOJmX16jh-DF5dccrd_OEGNZ0/edit?gid=749942651)
- API Endpoint
- See the [wiki page](https://github.com/hackforla/peopledepot/wiki/Create-Table-issues-data-gathering-workflow) for instructions.

### Description

Events that have been cancelled

### Data Fields

1. Copied from spreadsheet and checked off according to ERD. (unchecked items indicate a mismatch between ERD and spreadsheet, which requires a review)

Name | Type | FK Table | FK table Issue(s)
-- | -- | -- | --
event | int | event | #173 & #611
start_time | DateTimeField | |
cancelled_on | Date| |
reason | varchar | |
canceled by | int | user | #15 & #601

1. In ERD only (having items here indicates a mismatch, which requires a review)
- None

### FK status
- #15
- #173
- #601
- #611