Create Table: CheckIn

### Dependency
- ~#88~ formerly was depedent on this but we have gotten far enough along to make this issue now.

### Overview

We need to create the CheckIn table so that we can update a shared data store across hackforla.org, vrms, civictechjobs, and tables (onboarding) project.

#### Details

A table and a model are the same thing

### Action Items

- [x] identify and document table description (see spreadsheet under Resources)
- [x] 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)
- [ ] create a single model in Django (defining schema)
- [ ] 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).
- [ ] write an API end point
- [ ] write API unit tests
- [ ] document the endpoint

### 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

A user check ins to an event is tracked here

### Data Fields
- [ ] - Check to see if open issues in `FK status` below
- [ ] 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

Name | Type | FK Table | FK table Issue(s)
-- | -- | -- | --
event_occurrence | FK | EventOccurrence | #612
user | FK | user | #601
checked_in | timestamp | |

### FK status
- #612

### Associated Tables

1. Copied from spreadsheet and checked off according to ERD. (unchecked items indicate a mismatch between ERD and spreadsheet, which requires a review)
- [x] event (many-to-one) #612
- [x] user (many-to-one) #15, #601