Refactor location processing logic in create-event.js

As discussed in [PR #3762](https://github.com/airqo-platform/AirQo-api/pull/3762), the location processing logic for handling `cohort_id` and `grid_id` is duplicated across multiple methods in `src/device-registry/controllers/create-event.js`. Extracting this logic into a shared utility function would improve code maintainability and reduce redundancy.

This issue was requested by @Baalmart in [this comment](https://github.com/airqo-platform/AirQo-api/pull/3762#discussion_r1816465509).