Refactor Event Check-In to query path field instead of document name

**Refactor Event Check-In**
Currently the Event Check-In system makes a query for a document that has the name of the checkin path. For example if someone lands on `/checkin/mycoolevent` then we search for a document called `mycoolevent` to retrieve event information from. However, this causes challenges in the future since we cannot reuse the same link for a different event. Cloud Firestore prevents us from renaming documents and so querying based on document names means we are locked into using it for just that one event.

We would instead like to search for the document that has the `path_name` value set to the checkin link. This ensures that in the future can easily adapt a link to point to a different event simply by modifying the `path_name` value. Making this change also means that when we create a new event we no longer care about what the name of the document is.

**Task List**
- [ ] Refactor query [here](https://github.com/acmutd/portal-backend/blob/c38f090406ccf98531b6e3b887e5253c119c8de2/functions/src/application/portal.ts#L56). View how to make a firestore query [here](https://firebase.google.com/docs/firestore/query-data/queries)
- [ ] Change document write [here](https://github.com/acmutd/portal-backend/blob/c38f090406ccf98531b6e3b887e5253c119c8de2/functions/src/application/portal.ts#L86). Save the document ref from the previous task to reuse here instead of making a fresh query again.
- [ ] Change document write [here](https://github.com/acmutd/portal-backend/blob/c38f090406ccf98531b6e3b887e5253c119c8de2/functions/src/custom/event.ts#L67) when creating a new event. We no longer care about the document name so use `.collection(event_collection).add()`.