$id must be valid URI in json schema

## Problem

Since JSON Schema Draft 6, `$id` is required to be a valid URI reference as defined in [RFC3986, section 4.1](https://tools.ietf.org/html/rfc3986#section-4.1), which is either a URI (*e.g.,* `http://eventlogging.jupyter.org/event-schema`) or a relative reference ( *e.g.,* `/event-schema`). Currently, the `$id`s used in event schemas across different Jupyter project do not follow this rule:

[`binderhub.jupyter.org/launch`](https://github.com/jupyterhub/binderhub/blob/91e6bdd401b23a6cd19d097d2fa7158465fa02ce/binderhub/event-schemas/launch.json#L2)
[`hub.jupyter.org/server-action`](https://github.com/jupyterhub/jupyterhub/blob/477ee23ad396590482e4f1566ca6acef3961b1a9/jupyterhub/event-schemas/server-actions/v1.yaml#L1)

They lack the scheme part of the URI and thus are not valid URIs.

Due to this the schemas are not guaranteed to always work with JSON schema validators. One example is when trying to use `$ref`

```python
import jsonschema

schema = {
"$id": "hub.jupyter.org/example-schema",
"properties": {
"requester": {"$ref": "#/definitions/user"},
"target_user": {"$ref": "#/definitions/user"}
},
"definitions": {
"user": {
"type": "object",
"properties": {
"name": {"type": "string"},
"id": {"type": "string"}
}
}
}
}

instance = {
"requester": {"name": "a", "id": "1"},
"target_user": {"name": "b", "id": "2"}
}

jsonschema.validate(instance, schema)
```

This would fail

```python
jsonschema.exceptions.RefResolutionError: unknown url type: 'hub.jupyter.org/hub.jupyter.org/example-schema'
```

Change to `"$id": "http://hub.jupyter.org/example-schema"` or `"$id": "/example-schema"` and it validates fine.

There are potentially other undiscovered problems as well.

## Proposed solution

Either change `$id` to fully formed URI or keep it as a relative reference. The later is what's being used in MediaWiki eventlogging ([example](https://schema.wikimedia.org/repositories/primary/jsonschema/mediawiki/user/blocks-change/current.yaml))

Start by inspecting the referenced event schema files: binderhub/event-schemas/launch.json and jupyterhub/event-schemas/server-actions/v1.yaml, then reproduce the jsonschema validation example. Check the event schemas for `$id` values that are not valid URI references and update them consistently; done means the affected schemas validate and `$ref` resolution works.

Written by the indexing model from the issue text.