Relax rules in `EventSummary` DTO

**Describe the bug**
While working with a client's Command Centre instance I found that a number of `EventSummary` (while tailing the `/events/updates` endpoint)objects were failing to parse due to missing attributes or the attributes being too strict.

**To Reproduce**
Steps to reproduce the behavior:
1. Run the test_events.py
2. Or try and validate the response from `/api/events/updates` using `EventSummary`

**Expected behavior**
The validation should pass, instead the following fail:
- `operator.href` required and should be optional
- `card.id` is not sent down in the response
- `modified_item.href` is required and it not sent down
- `source.href` should be optional

**Screenshots**
NA

**Desktop (please complete the following information):**
NA

**Smartphone (please complete the following information):**
NA

**Additional context**
- See also #20
- Command Centre was running version `8.9`
- Before we go down debugging this, can we please make sure that the `follow` command is targeting the right URLs.