Slack Notifications When Low Server Disk Space (replace nagios)

### Feature Request

## Problem / Opportunity

## Proposal

### Breakdown

Implementation Details (for maintainers)

#### Related files

Refer to [this map of common Endpoints](https://docs.openlibrary.org/developers/backend/endpoints.html):
*

#### Requirements Checklist
Checklist of requirements that need to be satisfied in order for this issue to be closed:
* [ ]

#### Stakeholders

*

#### Instructions for Contributors

* **Before** [creating a new branch](https://docs.openlibrary.org/developers/tools/git.html#making-changes-and-creating-a-pull-request) or pushing up changes to a PR, please first [run these commands](https://docs.openlibrary.org/developers/tools/git.html#working-on-your-branch) to ensure your repository is up to date, as the pre-commit bot may add commits to your PRs upstream.